Download La subducción en México central- sismología, tectónica y vulcanología

Geos, Vol. 26, No. 1, Octubre, 2006
Sesión Especial
La Subducción en México Central:
Sismología, Tectónica y Vulcanología
Organizadores:
Xyoli Pérez Campos
Arturo Iglesias
Geos, Vol. 26, No. 1, Octubre, 2006
Geos, Vol. 26, No. 1, Octubre, 2006
SE01-1
LA SUBDUCCIÓN EN MÉXICO CENTRAL: SISMOLOGÍA, TECTÓNICA Y
VULCANOLOGÍA
SE01-2
CENOZOIC EVOLUTION OF THE CENTRAL PART
OF THE MEXICAN SUBDUCTION ZONE FROM
GEOLOGIC AND GEOPHYSICAL DATA - IN THE EVE
OF THE RESULT FROM THE “MASE” EXPERIMENT
A THERMO-MECHANICAL MODEL FOR
THE MICHOACAN SUBDUCTION ZONE
AND ASSOCIATED INTRA-ARC EXTENSION
Contreras Pérez Juan
Ferrari Luca
Centro de Geociencias, UNAM
División de Ciencias de la Tierra, CICESE
[email protected]
[email protected]
The Meso America Subduction Experiments (MASE), carried
our jointly by Caltech, UCLA and UNAM (Institute of Geophysics
and Center for Geoscience) is about to provide a detailed image
of the crust and upper mantle in the central part of the Mexican
subduction zone (Acapulco, Gro. – Huejutla, Hgo.). Preliminary
results show a subducted Cocos plate between the coast and
the volcanic front horizontal just beneath the upper plate Moho.
To the north, beneath the Trans-Mexican Volcanic Belt (TMVB),
seismicity is scarce or absent and the geometry of the subducted
plate is poorly defined. This part of the TMVB also displays a
large geochemical variability. This includes lavas with scarce to
none evidence of fluids from the subducting plate (OIB in Sierra
Chichinautzin) and lavas with slab melting signature (adakites
of Nevado de Toluca and Apan area) that coexist with the
more abundant product showing clear evidence of fluids from
the subduting plate. These peculiarities led several workers to
formulate models that depart from a classic subduction scenario
for the genesis of the TMVB. These include the presence of
a rootless mantle plume, the development of a continental rift,
a more or less abrupt increase of the subduction angle and
a detached slab. While waiting from the final results of the
MASE project the data available from potential methods, thermal
modeling and the geologic record of the TMVB provide some
constraints to evaluate these models.
Gravimetric and magnetotelluric data indicate that beneath the
TMVB the upper mantle has a relatively low density and high
temperatures/conductivity. Thermal modeling also indicates a low
viscosity and high temperature mantle beneath the arc. All the
above seems to indicate that the slab must increase rapidly its dip
beneath the volcanic front leaving space for the asthenospheric
mantle. The fate of the slab further to the north is unclear.
Regional tomographic models provide contradicting views: an
almost vertical slab or a detached slab. The geologic record of
the TMVB, however, provides some indications. From 20 to 10
Ma an andesitic arc migrated away from the trench, toward the
NE with the youngest and most inland centers having an adakitic
signature. This suggests that the subducted slab changed from
moderately dipping to flat between ~20 and 10 Ma. Between ~8
and 6.5 Ma a mafic volcanic episode is observed to the north of
the adakite belt. This occurred as a short-lived pulse of “intraplate”
lavas, part of a regional eastward propagating episode starting at
the mouth of the Gulf of California. Shortly after volcanism begins
a ~220 km return toward the trench, with regional ignimbrites and
silicic domes between 7.5 and 4 Ma and less differentiated lavas
in the Plio-Quaternary. The post 10 Ma evolution indicates that the
slab progressively increased its dip (rollback), consistently with
the presence of the convecting mantle beneath the whole late
Miocene-Quaternary arc. However, a lateral propagation of a slab
detachment may explain both the sudden change in composition
of volcanism and the triggering of the slab rollback in the late
Miocene.
The Trans-Mexican volcanic belt is a subduction-related arc
dissected by a field of seismically active normal faults clustered
in its western part. This field of normal faults is an enigmatic
feature of the Trans-Mexican volcanic belt and the nature of the
mechanism driving extension has been the subject of debate
for more than 25 years. These faults form en echelon arrays
and systems of nested faults aligned parallel to the axis of
the volcanic belt with a characteristic width of ~20 km. Fault
arrays seldom exceed 30 km in length and examples include
the Tepic-Zoacalco, Chapala, and Morelia-Acambay fault zones.
Moreover, crosscutting relations with basalt flows indicate that
these faults started to accrue displacement at ~5-6 My during a
period of high convergence rate between the North America and
Rivera plates.
The model consists of a 40 km-thick elastic plate (i.e., the
North America plate) sitting on top of Newtonian incompressible
fluid (upper mantle) forced in convection along the Wadati-Benioff
zone. The plate is allowed to undergo plasticity when deviatoric
stresses exceed the Mohr-Coulomb yield strength. The thermal
state of the subduction zone is also incorporated in the model,
given the strong dependence of the rehology of both mantle
and crust on temperature. Boundary conditions of the model
are consistent with heat-flow measurements, gravity modeling,
convergence rates derived from sea-floor magnetic anomalies, as
well as geological and seismological observations.
Model shows that extension in the arc is the direct result
of subduction due to viscous coupling between tectonic plates.
Numerical solutions indicate that positive changes in momentum
of the Rivera plate increase viscous drag along the base and
leading edge of North America resulting in downward bending of
the continental plate. This gives rise to tension 100-200 km inland
from the trench in good agreement with the location of the active
normal faults of the western Trans-Mexican volcanic belt.
Numerical solutions also develop shear bands (faults) that
self-organize forming horsts and nested grabens; the width of
the resulting structures scales with half the thickness of the
elastic plate, which also agrees with observations. Moreover,
when numerical experiments incorporate the topographic relief
of the volcanic arc an additional boundary effect arises in the
numerical experiments. The prescence of a plateau increases
tensional bending stresses within the volcanic arc by ~20% with
respect to surrounding lower areas. This could explain why normal
faults concentrate in the volcanic arc and their orientation parallel
to the axis of the arc.
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LA SUBDUCCIÓN EN MÉXICO CENTRAL: SISMOLOGÍA, TECTÓNICA Y
VULCANOLOGÍA
SE01-3
desplazamientos en cada celda se han calculando utilizando
las expresiones cerradas de Okada (1992), mientras que para
la inversión de los deslizamientos se utilizó un algoritmo de
cristalización simulada (Simulated Annealing).
CONTROLS OF SLAB STRUCTURE
BY LOW VISCOSITY WEDGES
Manea Vlad Constantin y Gurnis Mike
Seismological Laboratory, California Institute of Technology, USA
[email protected]
It is generally accepted that dehydration of subducting
lithosphere transport fluids into the mantle wedge, strongly
affecting the rheology and mechanical coupling. There are several
sources of fluids in subduction zones, from altered oceanic crust
and lithosphere to subducted pelagic sediments. The regions
beneath active volcanic arcs might hold viscosities lower than in
the oceanic asthenosphere, because of the abundant fluid inflow
from dehydration processes in subducting slabs. The maximum
depth extent of the hydrated mantle wedge (or low viscosity wedge
(LWV)) is controlled by the age of the incoming plate and the
convergence rate. The slab geometry evolution is related with the
balance between the gravitational body force and slab suction.
Thus, the time-space development of mantle wedge viscosity
would have significant influence on the subducting slab geometry
and therefore on the volcanic arc position. In this study we
investigate by numerical means the effect of the LVW on slab
evolution. Our models reveal that a well developed LVW, with
viscosity 0.5-0.05 times the astenospheric viscosity (10^20 Pa s),
decreases the slab dip from an initial dip of 30° to more than 50°,
depending on the viscosity of the mantle wedge. Steep slabs were
obtained in both cases of fixed-trench and trench-rollback models.
On the other hand, shallow LVW (<150 km depth) produced
perfectly flat slabs. The combination of the two models could
explain the onset of the flat slab in Central Mexico ~25 Ma, when
the Farallon plate broke into much younger Cocos plate. Then,
the younger Cocos slab dehydrated at shallower depths, and
therefore favoring only a partially developed LVW, which in turn
produced flat slab.
SE01-4
PATRONES DE DEFORMACIÓN EN LA
ZONA DE SUBDUCCIÓN DEL PACÍFICO
MEXICANO UTILIZANDO MEDICIONES GPS
1
2
Franco Sánchez Sara Ivonne , Iglesias Arturo ,
1
1
Kostoglodov Vladimir y Krishna Singh Shri
1
Instituto de Geofísica, UNAM
Seismological Laboratory, California Institute of Technology, USA
2
[email protected]
El presente trabajo tiene por objeto mostrar los patrones de
deformación cortical así como los resultados preliminares acerca
del grado de acoplamiento entre las placas tectónicas de Norte
América y Cocos a lo largo de la costa de los estados de Guerrero,
Oaxaca y Chiapas, el área de deformación activa debido al
proceso de subducción. Los vectores que describen la tasa
de acumulación de deformación han sido calculados utilizando
los datos de la red GPS del Instituto de Geofísica-Sismología
(SISMO-IGEOF), UNAM.
Para estimar el grado de acoplamiento, siguiendo un modelo
de “back-slip”, se ha llevado a cabo la inversión de las
deformaciones observadas en la superficie para determina
el deslizamiento “negativo” en la interfase de la placa. Los
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Geos, Vol. 26, No. 1, Octubre, 2006
Los resultados de la modelación en la región de
Guerrero-Oaxaca, muestran una zona con grado de acoplamiento
(#, relación entre el deslizamiento calculado para la interfase de
las placas y la velocidad de convergencia) #=1 cerca de la costa
y una zona de transición (# # 0.4) con una longitud aproximada
de 100 km. Para la zona ubicada al sur de Pinotepa, Oaxaca,
tenemos, nuevamente, una zona costera fuertemente acoplada
mientras que en la región del Istmo, al este de Huatulco, Oaxaca,
los modelos no muestran que exista acoplamiento.
Para el caso de Chiapas, debido a la escasa disponibilidad
de estaciones permanentes, no existe suficiente resolución para
constreñir un modelo, pero los resultados preliminares muestran
la existencia de una zona acoplada cerca de la trinchera.
Las zonas acopladas determinadas a partir de los modelos
coinciden con la región sismogénica delimitada por las áreas de
ruptura de los grandes terremotos registrados el siglo pasado en
la zona de subducción mexicana.
SE01-5
NONVOLCANIC TREMORS IN THE
MEXICAN SUBDUCTION ZONE
1
1
1
Kostoglodov Vladimir , Payero Juan , Mikumo Takeshi ,
1
1
2
Pérez Campos Xyoli , Iglesias Arturo y Clayton Robert W.
1
Instituto de Geofísica, UNAM
Seismological Laboratory, California Institute of Technology, USA
2
[email protected]
Nonvolcanic low frequency tremors (NVT) have been
discovered and studied recently in Japan and Cascadia
subduction zones. There are also reports on the NVT detected in
the Alaska subduction zone and deep beneath the San Andreas
Fault. The tremors activity is increasing during so-called silent
earthquakes (SQ) in Japan and Cascadia. NVT clusters also
migrate following the propagation of the SQ. Temporally and
spatially correlated SQ and NVT high activity are proposed
(Rogers & Dragert, 2003) to be called “Episodic Tremor and Slip”
events (ETS). Nevertheless, the origin of the NVT is still unclear.
There are two proposed mechanisms to explain NVT: the shearing
and fluid dehydration in the subducted oceanic crust. The studies
of NVT and SQ in different subduction zones are required to
understand the cause for these phenomena. We discovered a
number of NVT from daily spectrograms of continuous broad band
records at seismic stations of Servicio Seismológico Nacional
(SSN) an MASE project. The analyzed data cover a period
of 2001-2002 (SSN) when the largest SQ has occurred in
the Guerrero-Oaxaca region, and a steady-state interseismic
epoch of 2005-2006 (MASE). NVT occurred in the central part
of the Mexican subduction zone (Guerrero) at approximately
200 km from the coast. We can not accurately localize the
tremors because of sparse station coverage in 2001-2002 and an
unfavorable configuration of MASE network in 2005-2006. NVT
records in Mexico are very similar to those obtained in Cascadia
subduction zone. The tremors duration is of 10-60 min, and they
appear to travel at S-wave velocities. NVT depths are poorly
constrained but seem to be less than 40 km deep. However, we
could not notice any significant increase of NVT activity during the
2001-2002 SQ compared with an NVT activity for the “quiet” period
Geos, Vol. 26, No. 1, Octubre, 2006
LA SUBDUCCIÓN EN MÉXICO CENTRAL: SISMOLOGÍA, TECTÓNICA Y
VULCANOLOGÍA
of 2005-2006. This disagrees with the ETS concept and probably
related with a notably slower or longer lasting the 2001-2002 SQ
in Mexico in comparison with weeks-long Cascadia SQ.
SE01-7
A RECEIVER FUNCTION IMAGE ON HOW
COCOS SUBDUCTS BENEATH NORTH AMERICA
1
SE01-6
RESULTADOS PRELIMINARES DE LA TOMOGRAFÍA
DE ONDAS SUPERFICIALES DE ALTA
FRECUENCIA A LO LARGO DEL PERFIL DE MASE
1
1
2
1
Instituto de Geofísica, UNAM
Seismological Laboratory, California Institute of Technology, USA
3
Facultad de Ingeniería, UNAM
2
Iglesias Arturo , Clayton Robert W. , Pacheco Javier , Krishna
2
2
2
Singh Shri , Pérez Campos Xyoli y Valdés González Carlos
1
Seismological Laboratory, California Institute of Technology, USA
2
Departamento de Sismología, Instituto de Geofísica, UNAM
[email protected]
El arreglo MASE es un perfil de 100 estaciones sismológicas de
banda ancha registrando en modo continuo desde enero de 2005.
Esta línea comienza en Acapulco y se extiende, perpendicular a
la trinchera, hasta el estado de Veracruz.
En este trabajo presentamos los resultados preliminares de
la inversión tomográfica de ondas superficiales de velocidad
de grupo y la inversión, a lo largo del perfil, de las curvas de
dispersión locales obtenidas a través de dicha tomografía.
Dado que la geometría del arreglo no es la más adecuada
para llevar a cabo un estudio de este tipo, los datos fueron
complementados con datos de estaciones localizadas en el centro
y sur del país operadas por el SSN.
El proceso consiste en calcular curvas de dispersión de
velocidad de grupo de un conjunto de eventos bien localizados
(~50) a lo largo de las estaciones de MASE y del SSN. Estas
curvas mapean la variación de la velocidad de grupo promedio
para cada combinación evento-estación y fueron seleccionadas,
para el modo fundamental (5-50s), descartando los periodos
donde la grafica no es clara.
Para los periodos seleccionados, estas curvas fueron
sometidas, a una inversión tomográfica sobre una malla de 1/8 de
km por lado. El resultado es una imagen tomográfica de velocidad
de grupo para cada uno de los periodos seleccionados.
Usando una interpolación simple fueron calculados los valores
de velocidad de grupo para cada periodo estación construyendo
una curva de dispersión local para cada estación del perfil.
Finalmente la curva de dispersión local es invertida usando un
método de recristalización simulada obteniendo finalmente una
estructura de velocidades para cada estación. Los resultados
muestran variaciones de la estructura cortical a lo largo del perfil
y evidencia de la presencia de la cuña del manto debajo del eje
neovolcánico.
2
Pérez Campos Xyoli , Clayton Robert W. , Greene
1
3
2
Fernando , Espejo Lizbeth y Iglesias Arturo
[email protected]
Various seismological studies have mapped the subducted
Cocos plate beneath North America. They generally agree that
the slab dip varies from steep to shallow to steep from north
to south, with the shallow part near Acapulco. Since the central
segment appears to have a very different geometry, we expect
the behavior of the slab here to be also different. There are
diverse models for the fate of the slab after it increases its dip
angle and submerges in the mantle. Using receiver functions
along the MesoAmerican Suduction Experiment (MASE) array
(100 broadband seismometers from Acapulco, Gro. to Tempoal,
Ver.), we present preliminary results that illuminate North America
and Cocos plates along this profile. We are able to clearly follow
the slab up to 300 km away from the trench.
SE01-8 CARTEL
FLAT SUBDUCTION ZONE IN CENTRAL MEXICO:
CONSTRAINTS FROM AEROMAGNETIC ANOMALIES
Manea Marina y Manea Vlad Constantin
Seismological Laboratory, California Institute of Technology, USA
[email protected]
The aeromagnetic map of Mexico shows a magnetic “quiet
zone” in Guerrero and Oaxaca (Central Mexico), characterized
by a general lack of short-wavelength magnetic anomalies. In
order to investigate the magnetic quiet zone in relation with
the thermal sources, spectral analysis has been applied to the
aeromagnetic data. The results show the existence of deep
magnetic sources (30-40 km) which we consider to be the Currie
depth (corresponding to a temperature of 575-600°C). Above
the Curie temperature spontaneous magnetization vanishes and
the minerals exhibit only a small paramagnetic susceptibility.
Our estimates of magnetic basal depth are consistent with the
heat flow measurements in the area (20-30 mW/m2). In order to
explain such deep magnetic source and small heat flow estimates,
we infer the thermal structure associated with the subduction
of the Cocos plate beneath Central Mexico, using a finite
element approach. The modeling results show that the 575-600°C
isotherm is subhorisontal due to the flat slab regime in the area.
Also, the heat flow estimates from thermal models and spectral
analysis of aeromagnetic anomalies are in good agreement. We
conclude that the magnetic quiet zone is associated with a flat
slab subduction regime in Central Mexico, and proved to be
an important constraint for the thermal structure associated with
subduction zones.
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