Directory UMM :Data Elmu:jurnal:J-a:Journal Of Applied Geophysics:Vol44.Issue1.2000:
Journal of Applied Geophysics 44 Ž2000. 1–14
www.elsevier.nlrlocaterjappgeo
Magnetotelluric study of a plio-quaternary tectonic depression:
the Vilaric¸a basin žNE Portugal/
Fernando A. Monteiro Santos a,) , Eugenio
´ P. Almeida a,d , Antonio
´ Mateus b,
a,c,1
a
Hugo C. Matias
, Liliana Matos , Luıs
´ A. Mendes-Victor a
a
b
Departamento de Fısica
da UniÕersidade de Lisboa and Centro de Geofısica
da UniÕersidade de Lisboa,
´
´
R. Escola Politecnica,
58, 1250 Lisbon, Portugal
´
Departamento de Geologia da UniÕersidade de Lisboa, Edificio C2, Campo Grande, 1700 Lisbon, Portugal
c
Instituto Geologico
e Mineiro, Estrada da Portela, Zambujal, Apartado 7586, Portugal
´
d
Instituto Politecnico
de Tomar, Portugal
´
Received 7 August 1998; accepted 28 October 1999
Abstract
The Vilaric¸a basin, located northeast Portugal astride a major late-Variscan NNE–SSW reactivated strike-slip fault, is an
excellent example of interplate neotectonic activity whose development has been mainly interpreted as a result of left-lateral
displacement. Thirty magnetotelluric soundings were carried out in the Sta Comba da Vilaric¸arSampaio region Žnorthern of
the tectonic basin. in order to investigate the internal structure of the basin and its relationship with the main tectonic faults.
Distortions of the impedance tensors were studied using Groom–Bailey decomposition technique. The predominant regional
strike ŽN26E. is in good accordance with exposed geology and can be explained for the reactivation of previous structures.
Using two-dimensional inversion, three resistivity cross sections were obtained at north, center and south of the studied area.
The graben is revealed as a low resistivity Ž20–100 V m. structure due to the sedimentary filling. The increasing electrical
resistivity Žfrom 400 to 4000 V m. at depths greater than 2 km is related to the Hesperian basement rocks. The main faults,
which controlled the formation and evolution of the basin, are revealed by resistivity gradients within the upper crust.
q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Magnetotellurics; Tectonic basin; Electrical structure; Portugal
1. Introduction
)
Corresponding author. Fax: q351-1-395-3327; e-mail:
dfams@fc.ul.pt
1
Now at Petroprimo, Lisboa.
The Manteigas–Vilaric¸a–Braganc¸a leftlateral strike-slip fault zone, usually designated
as Vilaric¸a Fault Zone ŽVFZ. , is one of the
major elements of the late-Variscan fracture
network in NW Iberia Ž Fig. 1. . Both geological
and geomorphological criteria reveal that most
of the fault segments of the VFZ were reactivated during Meso and Cenozoic times. A left-
0926-9851r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 6 - 9 8 5 1 Ž 9 9 . 0 0 0 6 8 - 3
2
F.A. Monteiro Santos et al.r Journal of Applied Geophysics 44 (2000) 1–14
Fig. 1. Tectonic and geological sketch map of Vilaric¸a fault zone and MT sites location. Ž1. Alluvial sediments; Ž2. Autochthon domain; Ž3. Parautochthon domain;
Ž4. Lower Allochthon domain; Ž5. Syn-tectonic granites; Ž6. Tardi-tectonic granites; Ž7. Fault; Ž8. Fault Žprobable.; Ž9. Thrust fault.
F.A. Monteiro Santos et al.r Journal of Applied Geophysics 44 (2000) 1–14
lateral movement with upthrusting of the eastern
block towards WNW can be put in evidence in
many outcrops, representing the predominant
tectonic style of the reactivated VFZ segments
in Plio-Quaternary times; fault slip-rates ranging
from 0.2 to 0.5 mmryear are reported by Cabral
Ž1989. for the global Plio-Quaternary seismic
activity. A historical seismicity record associated to the VFZ demonstrates its present day
activity Že.g., Ribeiro, 1984; Moreira, 1985. .
The NNE–SSW Vilaric¸a depression Ž 20 km
long and 2–3 km wide., is located astride the
VFZ northern segment at Tras-os-Montes
and,
´
according to the available data, it is an excellent
example of intraplate neotectonic activity. Given
the geometric pattern of the mapped faults segments, as well as the nature and the distributions of the sedimentary units within the Vilaric¸a
basin, a pull-apart origin was proposed by Cabral
Ž1989. for this tectonic depression, whose development is related to the VFZ reactivation in
Plio-Quaternary times.
The VFZ northern segment has been the subject of several geological studies in the last
decades, which enabled the geometric characterisation of different tectonic structures, namely
the Vilaric¸a basin and the Bornes push-up along
the fault zone domain at Tras-os-Montes
region.
´
The transition between those two main structures can be observed in the Sta Comba da
Vilaric¸arSampaio region, and although the unquestionable scientific importance on delineating the interlinked discontinuities no significant
geophysical studies were performed until now.
In order to better understand the behaviour of
the active faults, as well as to precise the meaning and importance of some inferred tectonic
structures to the Vilaric¸a genesis Žsomething
impossible to do with the available geological
data., a MT survey was designed.
During 1996 and 1998, 30 magnetotelluric
ŽMT. soundings were carried out in the northern
tip of the Vilaric¸a basin in order to obtain an
image of the internal electrical resistivity distribution of the basin. Three profiles of MT soundings were inverted using a 2-D approximation.
3
The dimensionality of the structures was analysed undertaking Groom–Bailey Ž G-B. tensor
decompositions.
2. Geological setting
The Sta Comba da Vilaric¸arSampaio region
mainly comprises metasedimentary detrital rocks
of different lithostratigraphic units that belong
to the Autochthon, Parautochthon and Lower
Allochthon Domains of the Iberian Terrain in
NE Portugal ŽRibeiro, 1974; Ribeiro et al.,
1990.. It is also worth noting the presence of
syntectonic and late-tectonic granites that intrude, respectively, the autochthonous and allochthonous metasediments; these rocks belong
to major igneous bodies known as Vila Real–
Carvic¸ais and Sta Comba da Vilaric¸a batholiths
ŽRibeiro, 1974; Silva et al., 1989. .
In this region, it is possible to recognise
different tectonic structures that, according to
their displacement vectors, orientation, relative
chronology and geodynamic evolution, may be
grouped in two main systems. The first comprises segments of the regional thrusts andror
their subsidiary structures, generated during the
earlier Variscan deformation phases; bordering
the Parautochthon and Allochthon Domains,
they generally show a strong dip Ž) 608 towards N or NNE. and variable strike Ž NW–SE
to WNW–ESE. due to late-Variscan deformation events. The second system comprises
mainly subvertical strike-slip NNE-SSW faults
that bound the northern tip of the Vilaric¸a basin.
Although the polyphasic nature of the slip vector associated to these late tectonic accidents
can be easily identified on most of the mapped
fault surfaces, one could point out that a predominant left-lateral component is usually associated to a vertical displacement, responsible for
the relative down throwing of eastern or western
blocks. The detrital sediments that fill the northern tip of the Vilaric¸a basin can be grouped in
four main units of Pliocene to Quaternary age
ŽCabral, 1989 and references therein..
4
F.A. Monteiro Santos et al.r Journal of Applied Geophysics 44 (2000) 1–14
3. MT data processing and analysis
3.1. Data acquisition
In magnetotellurics, a frequency-dependent
transfer function, between electromagnetic field
components, is determined using simultaneous
measured variations of the electric ŽE. and magnetic Ž B. fields, on the surface of the earth. This
complex transfer function is usually designated
the impedance tensor. Apparent resistivity and
phase curves are calculated from the impedance
tensor elements Z i j with Z x y and Z y x representing the two basic components.
The MT data of this study were acquired
using a CNRS-GeoInstruments Ž France. system
with four channels with a recording frequency
ranging from 180 to 1r125 Hz. The measurement directions of the horizontal fields were
N26E and N116E, in accordance with the main
strike of the geological structures. The time
series have been processed and the impedance
tensors were obtained using a robust method
based on Egbert and Booker Ž 1986. . Data quality was strongly conditioned by the small geomagnetic activity at the time of the survey and
by cultural noise mainly by power lines crossing
the depression. Therefore, at some sites the
quality of the soundings is rather poor. Moreover, in the so-called ‘‘MT dead-band’’, due to
the weak signal level, data quality is generally
poor.
3.2. Tensor impedance decomposition
Distortions of the electric field by nearsurface inhomogeneities can have significant effects on the MT data leading to erroneous interpretation. Therefore, those distortions must be
recognised and corrected. Several techniques
have been proposed to deal with this problem.
In this study we applied the Groom–Bailey
ŽG-B. method which is one of the most current
methods ŽGroom and Bailey, 1991. . This method
evaluates the fit of the data to a 3D model of
galvanic distortion overlying a regional 2D
model through the analysis of three decomposition parameters: shear, twist and channeling.
The objective of that decomposition is also to
determine the dimensionality of the data, and, if
two-dimensional, to obtain a regional strike direction. The main steps of the process are as
follows: Ž1. first, a G-B decomposition is performed without any constraints; Ž 2. subsequently, the shear is constrained and Ž3. finally,
the shear and twist are constrained. Following
this procedure, the regional strikes and distortion parameters were obtained at each site for
all the frequencies. The validity of the model
hypothesis for the tensor decomposition was
tested through the misfit errors ŽGroom and
Bailey, 1991. . In general the results indicated
that the 3Dr2D decomposition model is valid in
the period range between 0.1 and 100 s. Below
0.1 s, the misfit is above the acceptable level.
Regional azimuth Ž strike. , shear, twist and
channeling parameters are given in Fig. 2 for
two sites. At site 9, only the strike was constrained to the measurement direction. The decomposition results show low decomposition
parameters and its roughly independence of the
frequency, revealing that the N26E strike is
supported by the data. At site 21 all the parameters were constrained. As can be noted, the
distortion parameters have low-frequency independent values and the N26E strike gives acceptable errors, for both cases, in the period
ranging from 0.1 to 100 s. These results are
broadly representative of the data acquired
within the graben.
3.3. Regional strike analysis
The average G-B regional strikes, obtained at
each site considering the data in the range between 0.1 and 100 s were used to obtain a
representative strike for the region. The dominant strike is N26E, which is also the direction
associated with the inactive segment of the Vi-
F.A. Monteiro Santos et al.r Journal of Applied Geophysics 44 (2000) 1–14
5
Fig. 2. Residual error, strike, twist, shear and channelling current azimuths obtained from Groom–Bailey decomposition of
data from two sites. At site 9 Žleft. only the strike was constrained to 268. At site 21 Žright., all parameters are constrained.
Note that directions are referred to field axis ŽN26E; N116E..
laric¸a fault. However, at some sites located in
the eastern domain of the basin Ž sites 24, 16, 17
and 36. , mainly in the vicinity of the NW–SE
thrust fault Žwhere 3-D responses are expected. ,
the strikes are ENE–WSW Ž from N58E to
N70E.. According to the principal strike the Eand B-polarizations refer to the directions of
electric fields parallel and perpendicular to the
N26E direction.
The regional strikes at each site can be interpreted in function of the reactivation of pre-existing structures generated during the late stages
of Variscan orogeny. In fact, field data reveals
that either the earlier semi-ductile shear zones
or the late-Variscan faults were subjected to
polyphasic reactivation in the course of the
Alpine Cycle. Therefore, ENE–WSW to NE–
SW structures are subsidiary to the major
NNE–SSW strike-slip faults, reflecting mainly
the local reactivation of earlier shear zones in a
low temperature brittle regime of deformation
ŽMateus et al., 1995. . A similar regional direction was found by Monteiro Santos et al. Ž1995.
in the Chaves basin located 60 km NW from the
VFZ, and related to another reactivated leftlateral strike-slip fault, the Penacova–Regua–
´
Verin fault zone.
4. 2D interpretation and discussion
4.1. InÕersion of phase data
Generally interpretations of MT data are
based primarily on the apparent resistivities.
6
F.A. Monteiro Santos et al.r Journal of Applied Geophysics 44 (2000) 1–14
However, more or less severe distortions, including the static-shift effect, of these curves
are common. Fortunately, such static effects do
not affect phase data. Therefore, we chose to
carry out a first interpretation based completely
on the inversion of phases. However, this interpretation should be taken with some care and
only in a qualitative sense.
Three profiles of MT soundings were inverted: the first, located northwards and including sites 30, 23, 4, 35, 22, 24, 20, 34 and 37;
one, in the centre, taking into account sites 30,
21, 27, 7, 28, 36, 16, 17 and 37 and the last,
southward comprising sites 29, 31, 9, 26, 8, 14,
13, 5, and 33. Stations 30 and 37 are located
relatively far from the graben, which is the main
target of this investigation. These soundings
were considered in the inversion of both, the
northern and central profiles, in order to obtain
information related to the surrounding structures.
The models obtained from the three profiles
are shown in Fig. 3. Only the first 6 km of the
crustal models were considered due to the poor
quality of the data at the longest periods. The
low resistivity zones Ž 25–100 V m. in the
central part of the profiles Žbetween km 8 and
10. are associated with the sedimentary fill of
the depression. In the northern profile the low
resistivity zone seems to continue subdued eastwards under a more resistive overburden. None
of those low resistivity zones extend bellow 1
km. At this depth the models suggest a resistive
basement. More specifically beneath sites 35,
22, 24, 20, 28, 36, 18 and 17 in the northern and
central profiles well defined resistive structures
are revealed at depths greater than 1 km. The
east and west boundaries of these high resistivity zones correlate very well with two main
NNE-SSW faults Žgoing through sites 17–34
and 7–35, see Fig. 1. .
The models show that there is a significant
break in uppermost crustal resistivity: more resistive blocks are revealed in the western part of
the northern and central profiles, at depths between 0.5 to 2 km. In the southern profile, the
Fig. 3. Resistivity models obtained from inversion of the
phase data. ŽA. Northern profile, ŽB. Central profile and
ŽC. southern profile. The line marked with a b shows the
approximate location of the basin.
resistive layer extends bellow 5 km. Eastwards
the models reveal a less resistive upper crust.
Due to the data quality at longer periods the
deeper structures are poorly resolved. Although
the inversion of the phases did not yield a
complete and totally satisfying model for the
region, electrical anomalous areas well corre-
F.A. Monteiro Santos et al.r Journal of Applied Geophysics 44 (2000) 1–14
7
Fig. 4. Resistivity models obtained from joint TE-TM mode data Žapparent resistivities and phases. 2-D inversion along, ŽA.
the northern line, ŽB. the central line and, ŽC. the southern line. The line marked with a b shows the approximate location of
the basin.
8
F.A. Monteiro Santos et al.r Journal of Applied Geophysics 44 (2000) 1–14
lated with the main geological structures were
detected.
4.2. InÕersion of the MT data
Usually, the E-polarisation data vary
smoothly from site to site, whereas the Bpolarisation data are discontinuous due to electrical resistivity changes, which appear preferentially in the W–E direction. In an attempt to
correct the data for static-shift we considered
the regional levels of the E-polarisation apparent resistivity curves at a period that samples
deeper than our region of interest in this study;
the first 5 km of the crust: we considered 1.3 s
as the period for deriving a regional level. The
mean value of the E-polarisation apparent resistivity data is about 500 V m. Therefore, both
apparent resistivity curves for each site were
simultaneously shifted, so that the E-polarisation data had a value of 500 V m at 1.3 s. The
static-shift correction is a difficult task that
cannot be carried out properly without additional geophysical information Že.g., from a
transient survey.. The adopted procedure of deriving a regional resistivity curve and shifting
the data to match this curve has been used by
several authors Že.g., Berdichevsky et al., 1989,
Vanyan et al., 1989; Jones and Dumas, 1993.
which is, obviously questionable. Such methodology was adopted here mainly because the
scarcities of geophysical database: any seismic,
electrical or gravity information is available.
In order to obtain a better approach to the
electrical resistivity distribution in the region
the three profiles of MT soundings were inverted considering apparent resistivities Ž corrected from static-shift. and phases. The inverse
models were obtained using the program presented by Mackie et al. Ž 1997. that, according to
the authors, find regularised solutions to the 2-D
inverse problem using Tikhonov regularisation.
The obtained electrical models are presented
in Fig. 4. Detailed interpretation of the models
requires independent information, including drill
hole data. At present, no drill hole is available
in the studied area. Owing to the scarce geophysical database of the region, the interpretation of the MT data is difficult. Therefore, our
interpretation of the resistivity features is mainly
supported by the exposed geology. All resistivity models evidence two important domains.
Ž1. The first one, up to depths of 1–2 km,
comprises mainly the sedimentary filling of the
tectonic basin and the fracturedrweathered areas of the adjoining metasedimentary rocks. This
domain presents strong lateral resistivity contrasts with the lowest resistivities related to the
sedimentary units Ž 20–100 V m.. The resistivity distribution in the shallow domain Žup to
depths of 500 m. is strongly affected by the
network of tectonic features, distribution and
quantity of clay and water content, as well as by
stratification of the lithostratigrafic units.
Ž2. The second domain, related to the older
geological formations at depths greater than 2
km, which are represented by high resistivity
structures. The deeper levels of the Autochthonous, show an average resistivity ranging from 1000 to 4000 V m. At depths greater
than 2–3 km, the gneissic basement with resistivity greater than 4000 V m, is revealed at the
western part of the profiles.
The low resistivity areas associated with the
sedimentary fill of the depression show different
expressions: in the northern profile that zone
Žbeneath sites 4 and 35. seems to continue
subdued in depth Ž1 to 2 km. between sites 22
and 24, as revealed in the model obtained from
phases. However, the characteristics of that conductive structure is not well defined due to the
lack of data between those sites. The MT model
from the central profile suggests that the sedimentary filling Žbeneath sites 27, 7 and 28. is
Fig. 5. MT data for selected sites Žsymbols. and model responses Žsolid lines. from the inverse models. ŽA. From northern
profile; ŽB. from central profile and ŽC. from southern profile.
F.A. Monteiro Santos et al.r Journal of Applied Geophysics 44 (2000) 1–14
9
F.A. Monteiro Santos et al.r Journal of Applied Geophysics 44 (2000) 1–14
Fig. 5 Žcontinued..
10
11
Fig. 5 Žcontinued..
F.A. Monteiro Santos et al.r Journal of Applied Geophysics 44 (2000) 1–14
12
F.A. Monteiro Santos et al.r Journal of Applied Geophysics 44 (2000) 1–14
less thick in that region Ž thickness less than 1
km..
In the last model, corresponding to the southern profile, a thick Ž- 3 km. low resistivity
zone is clearly imaged beneath sites 8 and 14.
The three resistivity models are also characterised by a general increase in resistivity beneath sites located westwards of the depression
Žsites 30, 23, 21, 29, 31 and 9. and less resistivity structures eastwards.
The main faults associated with the generation of the basin Ž see Fig. 1. are well displayed
in the models: to the west, at depths greater than
3 km beneath sites 23, 4, 21 and 9; to the east,
the faults appear related to the resistivity contrast evidenced beneath sites 28, 36, 16 and 13.
Comparing the resistivity models obtained
from phases with those estimated using all MT
data, we found the same general behaviour on
the low-resistivity connected to the basin and
the high resistivity associated with old formations. The joint inversion of the apparent resistivities and phases allow a better definition of
the depth of the electrical resistivity boundaries.
Owing to our results the static-shift correction
methodology used in this study seems acceptable.
The models put into evidence the deep
three-dimensional geometry already suggested
by the complex superficial geology and shows
the need of three-dimensional modelling for a
better understand of the connection between the
structures. This must be the next step in the
interpretation of the magnetotelluric data set
acquired in the Vilaric¸a region. The two-dimensional modelling is clearly a first approach.
In Fig. 5, the experimental data and the corresponding fit of the inverse model responses at
representative sites along each of the profiles
are displayed. For most sites, the fit is relatively
good at shorter periods. However, the misfit
observed at some sites in the period range of
0.01 to 0.1 s may exhibit the need for three-dimensional modelling. At longer periods, however, the data are scattered and the fit is not so
good. The global rms for the profiles were 4.9
Žnorthern., 5.2 Žcenter. and 3.9 Žsouthern., respectively.
Possible geological interpretations of the main
characteristics revealed by the central and south
resistivity models are presented in Fig. 6. These
interpretations that combine the geophysical and
geological available information, put in evidence the asymmetric character of the basin,
ascribable to the simultaneous left-lateral
strike-slip and normal displacement of the faults
that bound the eastern and western sides of the
basin, respectively. According to the transformnormal extension model type, proposed by
Ben-Avraham and Zoback Ž1992. , the basin
Fig. 6. Interpreted geological cross section for A–B and
C–D lines. Numbers on the topographic line represent the
approximate location of the MT sites. The line marked
with a b shows the approximate location of the basin.
F.A. Monteiro Santos et al.r Journal of Applied Geophysics 44 (2000) 1–14
13
subsidence can thus be due to the extension in a
direction normal to the regional strike of the
fault at the same time that strike-slip movement
is taking place. This behaviour can be explained
if the far-field maximum principal stress direction is less than 458 relatively to the strike of the
fault zone: in such case a divergent motion can
occur ŽBen-Avraham and Zoback, 1992. . The
principal stress pattern, during Plio-Quaternary
times in northern Portugal Ž Cabral, 1989. is
consistent with this interpretation.
eastern part of the basin tilting north and southwards.
Ž5. At depths of 2 km, in the western part of
the studied area, the resistivity increases up to
values greater than 4000 V m. We interpret the
deepermost part of this layer as the Iberian
gneiss basement.
Ž6. The resistivity gradients revealed in the
upper crust were associated with the main fault
that controls the formation and evolution of the
depression in Quaternary times.
5. Conclusions
Acknowledgements
Two-dimensional inversion of MT data collected across the VFZ, allowed a characterization the electrical structure of the main geological units present in the studied area. Although
the 2-D modeling seems to be only a rough
approximation to the complex resistivity features of the region, it can give a first idea of the
resistivity distribution within the basin. The
general behaviour of the data indicates that a
3-D modeling might explain all the features of
the data. The main characteristics of the 2-D
estimated models and the interpretation of its
features, are as follows.
Ž1. The uppermost lithologies of the Parautochthonous show an average resistivity of
100–200 V m.
Ž2. The transition zone between Lower Allochthonous–Parautochthonous and the Parautochthonous is represented by a 350 m thickness layer with a resistivity of 200–500 V m.
Ž3. The autochthonous metasediments Ždominated by quartzites, pelites and graywackes. ,
display distinct resistivity values, ranging from
500 to 4000 V m, defining different layers.
Ž4. The sedimentary filling of the tectonic
basin is a good conductor, presenting average
resistivities of 20–100 V m. The low resistivity
of this zone is mainly due to the water content
of the sedimentary units. The models suggest
that this conductor is deeper within the north-
This study was supported by FCT ŽFundac¸ao
˜
para a Ciencia
e Tecnologia. ŽProject No.
ˆ
PBICTrCrCTAr2123r95. . The authors thank
Dr. G. Vasseur and an anonymous reviewer for
critical and useful review.
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www.elsevier.nlrlocaterjappgeo
Magnetotelluric study of a plio-quaternary tectonic depression:
the Vilaric¸a basin žNE Portugal/
Fernando A. Monteiro Santos a,) , Eugenio
´ P. Almeida a,d , Antonio
´ Mateus b,
a,c,1
a
Hugo C. Matias
, Liliana Matos , Luıs
´ A. Mendes-Victor a
a
b
Departamento de Fısica
da UniÕersidade de Lisboa and Centro de Geofısica
da UniÕersidade de Lisboa,
´
´
R. Escola Politecnica,
58, 1250 Lisbon, Portugal
´
Departamento de Geologia da UniÕersidade de Lisboa, Edificio C2, Campo Grande, 1700 Lisbon, Portugal
c
Instituto Geologico
e Mineiro, Estrada da Portela, Zambujal, Apartado 7586, Portugal
´
d
Instituto Politecnico
de Tomar, Portugal
´
Received 7 August 1998; accepted 28 October 1999
Abstract
The Vilaric¸a basin, located northeast Portugal astride a major late-Variscan NNE–SSW reactivated strike-slip fault, is an
excellent example of interplate neotectonic activity whose development has been mainly interpreted as a result of left-lateral
displacement. Thirty magnetotelluric soundings were carried out in the Sta Comba da Vilaric¸arSampaio region Žnorthern of
the tectonic basin. in order to investigate the internal structure of the basin and its relationship with the main tectonic faults.
Distortions of the impedance tensors were studied using Groom–Bailey decomposition technique. The predominant regional
strike ŽN26E. is in good accordance with exposed geology and can be explained for the reactivation of previous structures.
Using two-dimensional inversion, three resistivity cross sections were obtained at north, center and south of the studied area.
The graben is revealed as a low resistivity Ž20–100 V m. structure due to the sedimentary filling. The increasing electrical
resistivity Žfrom 400 to 4000 V m. at depths greater than 2 km is related to the Hesperian basement rocks. The main faults,
which controlled the formation and evolution of the basin, are revealed by resistivity gradients within the upper crust.
q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Magnetotellurics; Tectonic basin; Electrical structure; Portugal
1. Introduction
)
Corresponding author. Fax: q351-1-395-3327; e-mail:
dfams@fc.ul.pt
1
Now at Petroprimo, Lisboa.
The Manteigas–Vilaric¸a–Braganc¸a leftlateral strike-slip fault zone, usually designated
as Vilaric¸a Fault Zone ŽVFZ. , is one of the
major elements of the late-Variscan fracture
network in NW Iberia Ž Fig. 1. . Both geological
and geomorphological criteria reveal that most
of the fault segments of the VFZ were reactivated during Meso and Cenozoic times. A left-
0926-9851r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 6 - 9 8 5 1 Ž 9 9 . 0 0 0 6 8 - 3
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F.A. Monteiro Santos et al.r Journal of Applied Geophysics 44 (2000) 1–14
Fig. 1. Tectonic and geological sketch map of Vilaric¸a fault zone and MT sites location. Ž1. Alluvial sediments; Ž2. Autochthon domain; Ž3. Parautochthon domain;
Ž4. Lower Allochthon domain; Ž5. Syn-tectonic granites; Ž6. Tardi-tectonic granites; Ž7. Fault; Ž8. Fault Žprobable.; Ž9. Thrust fault.
F.A. Monteiro Santos et al.r Journal of Applied Geophysics 44 (2000) 1–14
lateral movement with upthrusting of the eastern
block towards WNW can be put in evidence in
many outcrops, representing the predominant
tectonic style of the reactivated VFZ segments
in Plio-Quaternary times; fault slip-rates ranging
from 0.2 to 0.5 mmryear are reported by Cabral
Ž1989. for the global Plio-Quaternary seismic
activity. A historical seismicity record associated to the VFZ demonstrates its present day
activity Že.g., Ribeiro, 1984; Moreira, 1985. .
The NNE–SSW Vilaric¸a depression Ž 20 km
long and 2–3 km wide., is located astride the
VFZ northern segment at Tras-os-Montes
and,
´
according to the available data, it is an excellent
example of intraplate neotectonic activity. Given
the geometric pattern of the mapped faults segments, as well as the nature and the distributions of the sedimentary units within the Vilaric¸a
basin, a pull-apart origin was proposed by Cabral
Ž1989. for this tectonic depression, whose development is related to the VFZ reactivation in
Plio-Quaternary times.
The VFZ northern segment has been the subject of several geological studies in the last
decades, which enabled the geometric characterisation of different tectonic structures, namely
the Vilaric¸a basin and the Bornes push-up along
the fault zone domain at Tras-os-Montes
region.
´
The transition between those two main structures can be observed in the Sta Comba da
Vilaric¸arSampaio region, and although the unquestionable scientific importance on delineating the interlinked discontinuities no significant
geophysical studies were performed until now.
In order to better understand the behaviour of
the active faults, as well as to precise the meaning and importance of some inferred tectonic
structures to the Vilaric¸a genesis Žsomething
impossible to do with the available geological
data., a MT survey was designed.
During 1996 and 1998, 30 magnetotelluric
ŽMT. soundings were carried out in the northern
tip of the Vilaric¸a basin in order to obtain an
image of the internal electrical resistivity distribution of the basin. Three profiles of MT soundings were inverted using a 2-D approximation.
3
The dimensionality of the structures was analysed undertaking Groom–Bailey Ž G-B. tensor
decompositions.
2. Geological setting
The Sta Comba da Vilaric¸arSampaio region
mainly comprises metasedimentary detrital rocks
of different lithostratigraphic units that belong
to the Autochthon, Parautochthon and Lower
Allochthon Domains of the Iberian Terrain in
NE Portugal ŽRibeiro, 1974; Ribeiro et al.,
1990.. It is also worth noting the presence of
syntectonic and late-tectonic granites that intrude, respectively, the autochthonous and allochthonous metasediments; these rocks belong
to major igneous bodies known as Vila Real–
Carvic¸ais and Sta Comba da Vilaric¸a batholiths
ŽRibeiro, 1974; Silva et al., 1989. .
In this region, it is possible to recognise
different tectonic structures that, according to
their displacement vectors, orientation, relative
chronology and geodynamic evolution, may be
grouped in two main systems. The first comprises segments of the regional thrusts andror
their subsidiary structures, generated during the
earlier Variscan deformation phases; bordering
the Parautochthon and Allochthon Domains,
they generally show a strong dip Ž) 608 towards N or NNE. and variable strike Ž NW–SE
to WNW–ESE. due to late-Variscan deformation events. The second system comprises
mainly subvertical strike-slip NNE-SSW faults
that bound the northern tip of the Vilaric¸a basin.
Although the polyphasic nature of the slip vector associated to these late tectonic accidents
can be easily identified on most of the mapped
fault surfaces, one could point out that a predominant left-lateral component is usually associated to a vertical displacement, responsible for
the relative down throwing of eastern or western
blocks. The detrital sediments that fill the northern tip of the Vilaric¸a basin can be grouped in
four main units of Pliocene to Quaternary age
ŽCabral, 1989 and references therein..
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F.A. Monteiro Santos et al.r Journal of Applied Geophysics 44 (2000) 1–14
3. MT data processing and analysis
3.1. Data acquisition
In magnetotellurics, a frequency-dependent
transfer function, between electromagnetic field
components, is determined using simultaneous
measured variations of the electric ŽE. and magnetic Ž B. fields, on the surface of the earth. This
complex transfer function is usually designated
the impedance tensor. Apparent resistivity and
phase curves are calculated from the impedance
tensor elements Z i j with Z x y and Z y x representing the two basic components.
The MT data of this study were acquired
using a CNRS-GeoInstruments Ž France. system
with four channels with a recording frequency
ranging from 180 to 1r125 Hz. The measurement directions of the horizontal fields were
N26E and N116E, in accordance with the main
strike of the geological structures. The time
series have been processed and the impedance
tensors were obtained using a robust method
based on Egbert and Booker Ž 1986. . Data quality was strongly conditioned by the small geomagnetic activity at the time of the survey and
by cultural noise mainly by power lines crossing
the depression. Therefore, at some sites the
quality of the soundings is rather poor. Moreover, in the so-called ‘‘MT dead-band’’, due to
the weak signal level, data quality is generally
poor.
3.2. Tensor impedance decomposition
Distortions of the electric field by nearsurface inhomogeneities can have significant effects on the MT data leading to erroneous interpretation. Therefore, those distortions must be
recognised and corrected. Several techniques
have been proposed to deal with this problem.
In this study we applied the Groom–Bailey
ŽG-B. method which is one of the most current
methods ŽGroom and Bailey, 1991. . This method
evaluates the fit of the data to a 3D model of
galvanic distortion overlying a regional 2D
model through the analysis of three decomposition parameters: shear, twist and channeling.
The objective of that decomposition is also to
determine the dimensionality of the data, and, if
two-dimensional, to obtain a regional strike direction. The main steps of the process are as
follows: Ž1. first, a G-B decomposition is performed without any constraints; Ž 2. subsequently, the shear is constrained and Ž3. finally,
the shear and twist are constrained. Following
this procedure, the regional strikes and distortion parameters were obtained at each site for
all the frequencies. The validity of the model
hypothesis for the tensor decomposition was
tested through the misfit errors ŽGroom and
Bailey, 1991. . In general the results indicated
that the 3Dr2D decomposition model is valid in
the period range between 0.1 and 100 s. Below
0.1 s, the misfit is above the acceptable level.
Regional azimuth Ž strike. , shear, twist and
channeling parameters are given in Fig. 2 for
two sites. At site 9, only the strike was constrained to the measurement direction. The decomposition results show low decomposition
parameters and its roughly independence of the
frequency, revealing that the N26E strike is
supported by the data. At site 21 all the parameters were constrained. As can be noted, the
distortion parameters have low-frequency independent values and the N26E strike gives acceptable errors, for both cases, in the period
ranging from 0.1 to 100 s. These results are
broadly representative of the data acquired
within the graben.
3.3. Regional strike analysis
The average G-B regional strikes, obtained at
each site considering the data in the range between 0.1 and 100 s were used to obtain a
representative strike for the region. The dominant strike is N26E, which is also the direction
associated with the inactive segment of the Vi-
F.A. Monteiro Santos et al.r Journal of Applied Geophysics 44 (2000) 1–14
5
Fig. 2. Residual error, strike, twist, shear and channelling current azimuths obtained from Groom–Bailey decomposition of
data from two sites. At site 9 Žleft. only the strike was constrained to 268. At site 21 Žright., all parameters are constrained.
Note that directions are referred to field axis ŽN26E; N116E..
laric¸a fault. However, at some sites located in
the eastern domain of the basin Ž sites 24, 16, 17
and 36. , mainly in the vicinity of the NW–SE
thrust fault Žwhere 3-D responses are expected. ,
the strikes are ENE–WSW Ž from N58E to
N70E.. According to the principal strike the Eand B-polarizations refer to the directions of
electric fields parallel and perpendicular to the
N26E direction.
The regional strikes at each site can be interpreted in function of the reactivation of pre-existing structures generated during the late stages
of Variscan orogeny. In fact, field data reveals
that either the earlier semi-ductile shear zones
or the late-Variscan faults were subjected to
polyphasic reactivation in the course of the
Alpine Cycle. Therefore, ENE–WSW to NE–
SW structures are subsidiary to the major
NNE–SSW strike-slip faults, reflecting mainly
the local reactivation of earlier shear zones in a
low temperature brittle regime of deformation
ŽMateus et al., 1995. . A similar regional direction was found by Monteiro Santos et al. Ž1995.
in the Chaves basin located 60 km NW from the
VFZ, and related to another reactivated leftlateral strike-slip fault, the Penacova–Regua–
´
Verin fault zone.
4. 2D interpretation and discussion
4.1. InÕersion of phase data
Generally interpretations of MT data are
based primarily on the apparent resistivities.
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F.A. Monteiro Santos et al.r Journal of Applied Geophysics 44 (2000) 1–14
However, more or less severe distortions, including the static-shift effect, of these curves
are common. Fortunately, such static effects do
not affect phase data. Therefore, we chose to
carry out a first interpretation based completely
on the inversion of phases. However, this interpretation should be taken with some care and
only in a qualitative sense.
Three profiles of MT soundings were inverted: the first, located northwards and including sites 30, 23, 4, 35, 22, 24, 20, 34 and 37;
one, in the centre, taking into account sites 30,
21, 27, 7, 28, 36, 16, 17 and 37 and the last,
southward comprising sites 29, 31, 9, 26, 8, 14,
13, 5, and 33. Stations 30 and 37 are located
relatively far from the graben, which is the main
target of this investigation. These soundings
were considered in the inversion of both, the
northern and central profiles, in order to obtain
information related to the surrounding structures.
The models obtained from the three profiles
are shown in Fig. 3. Only the first 6 km of the
crustal models were considered due to the poor
quality of the data at the longest periods. The
low resistivity zones Ž 25–100 V m. in the
central part of the profiles Žbetween km 8 and
10. are associated with the sedimentary fill of
the depression. In the northern profile the low
resistivity zone seems to continue subdued eastwards under a more resistive overburden. None
of those low resistivity zones extend bellow 1
km. At this depth the models suggest a resistive
basement. More specifically beneath sites 35,
22, 24, 20, 28, 36, 18 and 17 in the northern and
central profiles well defined resistive structures
are revealed at depths greater than 1 km. The
east and west boundaries of these high resistivity zones correlate very well with two main
NNE-SSW faults Žgoing through sites 17–34
and 7–35, see Fig. 1. .
The models show that there is a significant
break in uppermost crustal resistivity: more resistive blocks are revealed in the western part of
the northern and central profiles, at depths between 0.5 to 2 km. In the southern profile, the
Fig. 3. Resistivity models obtained from inversion of the
phase data. ŽA. Northern profile, ŽB. Central profile and
ŽC. southern profile. The line marked with a b shows the
approximate location of the basin.
resistive layer extends bellow 5 km. Eastwards
the models reveal a less resistive upper crust.
Due to the data quality at longer periods the
deeper structures are poorly resolved. Although
the inversion of the phases did not yield a
complete and totally satisfying model for the
region, electrical anomalous areas well corre-
F.A. Monteiro Santos et al.r Journal of Applied Geophysics 44 (2000) 1–14
7
Fig. 4. Resistivity models obtained from joint TE-TM mode data Žapparent resistivities and phases. 2-D inversion along, ŽA.
the northern line, ŽB. the central line and, ŽC. the southern line. The line marked with a b shows the approximate location of
the basin.
8
F.A. Monteiro Santos et al.r Journal of Applied Geophysics 44 (2000) 1–14
lated with the main geological structures were
detected.
4.2. InÕersion of the MT data
Usually, the E-polarisation data vary
smoothly from site to site, whereas the Bpolarisation data are discontinuous due to electrical resistivity changes, which appear preferentially in the W–E direction. In an attempt to
correct the data for static-shift we considered
the regional levels of the E-polarisation apparent resistivity curves at a period that samples
deeper than our region of interest in this study;
the first 5 km of the crust: we considered 1.3 s
as the period for deriving a regional level. The
mean value of the E-polarisation apparent resistivity data is about 500 V m. Therefore, both
apparent resistivity curves for each site were
simultaneously shifted, so that the E-polarisation data had a value of 500 V m at 1.3 s. The
static-shift correction is a difficult task that
cannot be carried out properly without additional geophysical information Že.g., from a
transient survey.. The adopted procedure of deriving a regional resistivity curve and shifting
the data to match this curve has been used by
several authors Že.g., Berdichevsky et al., 1989,
Vanyan et al., 1989; Jones and Dumas, 1993.
which is, obviously questionable. Such methodology was adopted here mainly because the
scarcities of geophysical database: any seismic,
electrical or gravity information is available.
In order to obtain a better approach to the
electrical resistivity distribution in the region
the three profiles of MT soundings were inverted considering apparent resistivities Ž corrected from static-shift. and phases. The inverse
models were obtained using the program presented by Mackie et al. Ž 1997. that, according to
the authors, find regularised solutions to the 2-D
inverse problem using Tikhonov regularisation.
The obtained electrical models are presented
in Fig. 4. Detailed interpretation of the models
requires independent information, including drill
hole data. At present, no drill hole is available
in the studied area. Owing to the scarce geophysical database of the region, the interpretation of the MT data is difficult. Therefore, our
interpretation of the resistivity features is mainly
supported by the exposed geology. All resistivity models evidence two important domains.
Ž1. The first one, up to depths of 1–2 km,
comprises mainly the sedimentary filling of the
tectonic basin and the fracturedrweathered areas of the adjoining metasedimentary rocks. This
domain presents strong lateral resistivity contrasts with the lowest resistivities related to the
sedimentary units Ž 20–100 V m.. The resistivity distribution in the shallow domain Žup to
depths of 500 m. is strongly affected by the
network of tectonic features, distribution and
quantity of clay and water content, as well as by
stratification of the lithostratigrafic units.
Ž2. The second domain, related to the older
geological formations at depths greater than 2
km, which are represented by high resistivity
structures. The deeper levels of the Autochthonous, show an average resistivity ranging from 1000 to 4000 V m. At depths greater
than 2–3 km, the gneissic basement with resistivity greater than 4000 V m, is revealed at the
western part of the profiles.
The low resistivity areas associated with the
sedimentary fill of the depression show different
expressions: in the northern profile that zone
Žbeneath sites 4 and 35. seems to continue
subdued in depth Ž1 to 2 km. between sites 22
and 24, as revealed in the model obtained from
phases. However, the characteristics of that conductive structure is not well defined due to the
lack of data between those sites. The MT model
from the central profile suggests that the sedimentary filling Žbeneath sites 27, 7 and 28. is
Fig. 5. MT data for selected sites Žsymbols. and model responses Žsolid lines. from the inverse models. ŽA. From northern
profile; ŽB. from central profile and ŽC. from southern profile.
F.A. Monteiro Santos et al.r Journal of Applied Geophysics 44 (2000) 1–14
9
F.A. Monteiro Santos et al.r Journal of Applied Geophysics 44 (2000) 1–14
Fig. 5 Žcontinued..
10
11
Fig. 5 Žcontinued..
F.A. Monteiro Santos et al.r Journal of Applied Geophysics 44 (2000) 1–14
12
F.A. Monteiro Santos et al.r Journal of Applied Geophysics 44 (2000) 1–14
less thick in that region Ž thickness less than 1
km..
In the last model, corresponding to the southern profile, a thick Ž- 3 km. low resistivity
zone is clearly imaged beneath sites 8 and 14.
The three resistivity models are also characterised by a general increase in resistivity beneath sites located westwards of the depression
Žsites 30, 23, 21, 29, 31 and 9. and less resistivity structures eastwards.
The main faults associated with the generation of the basin Ž see Fig. 1. are well displayed
in the models: to the west, at depths greater than
3 km beneath sites 23, 4, 21 and 9; to the east,
the faults appear related to the resistivity contrast evidenced beneath sites 28, 36, 16 and 13.
Comparing the resistivity models obtained
from phases with those estimated using all MT
data, we found the same general behaviour on
the low-resistivity connected to the basin and
the high resistivity associated with old formations. The joint inversion of the apparent resistivities and phases allow a better definition of
the depth of the electrical resistivity boundaries.
Owing to our results the static-shift correction
methodology used in this study seems acceptable.
The models put into evidence the deep
three-dimensional geometry already suggested
by the complex superficial geology and shows
the need of three-dimensional modelling for a
better understand of the connection between the
structures. This must be the next step in the
interpretation of the magnetotelluric data set
acquired in the Vilaric¸a region. The two-dimensional modelling is clearly a first approach.
In Fig. 5, the experimental data and the corresponding fit of the inverse model responses at
representative sites along each of the profiles
are displayed. For most sites, the fit is relatively
good at shorter periods. However, the misfit
observed at some sites in the period range of
0.01 to 0.1 s may exhibit the need for three-dimensional modelling. At longer periods, however, the data are scattered and the fit is not so
good. The global rms for the profiles were 4.9
Žnorthern., 5.2 Žcenter. and 3.9 Žsouthern., respectively.
Possible geological interpretations of the main
characteristics revealed by the central and south
resistivity models are presented in Fig. 6. These
interpretations that combine the geophysical and
geological available information, put in evidence the asymmetric character of the basin,
ascribable to the simultaneous left-lateral
strike-slip and normal displacement of the faults
that bound the eastern and western sides of the
basin, respectively. According to the transformnormal extension model type, proposed by
Ben-Avraham and Zoback Ž1992. , the basin
Fig. 6. Interpreted geological cross section for A–B and
C–D lines. Numbers on the topographic line represent the
approximate location of the MT sites. The line marked
with a b shows the approximate location of the basin.
F.A. Monteiro Santos et al.r Journal of Applied Geophysics 44 (2000) 1–14
13
subsidence can thus be due to the extension in a
direction normal to the regional strike of the
fault at the same time that strike-slip movement
is taking place. This behaviour can be explained
if the far-field maximum principal stress direction is less than 458 relatively to the strike of the
fault zone: in such case a divergent motion can
occur ŽBen-Avraham and Zoback, 1992. . The
principal stress pattern, during Plio-Quaternary
times in northern Portugal Ž Cabral, 1989. is
consistent with this interpretation.
eastern part of the basin tilting north and southwards.
Ž5. At depths of 2 km, in the western part of
the studied area, the resistivity increases up to
values greater than 4000 V m. We interpret the
deepermost part of this layer as the Iberian
gneiss basement.
Ž6. The resistivity gradients revealed in the
upper crust were associated with the main fault
that controls the formation and evolution of the
depression in Quaternary times.
5. Conclusions
Acknowledgements
Two-dimensional inversion of MT data collected across the VFZ, allowed a characterization the electrical structure of the main geological units present in the studied area. Although
the 2-D modeling seems to be only a rough
approximation to the complex resistivity features of the region, it can give a first idea of the
resistivity distribution within the basin. The
general behaviour of the data indicates that a
3-D modeling might explain all the features of
the data. The main characteristics of the 2-D
estimated models and the interpretation of its
features, are as follows.
Ž1. The uppermost lithologies of the Parautochthonous show an average resistivity of
100–200 V m.
Ž2. The transition zone between Lower Allochthonous–Parautochthonous and the Parautochthonous is represented by a 350 m thickness layer with a resistivity of 200–500 V m.
Ž3. The autochthonous metasediments Ždominated by quartzites, pelites and graywackes. ,
display distinct resistivity values, ranging from
500 to 4000 V m, defining different layers.
Ž4. The sedimentary filling of the tectonic
basin is a good conductor, presenting average
resistivities of 20–100 V m. The low resistivity
of this zone is mainly due to the water content
of the sedimentary units. The models suggest
that this conductor is deeper within the north-
This study was supported by FCT ŽFundac¸ao
˜
para a Ciencia
e Tecnologia. ŽProject No.
ˆ
PBICTrCrCTAr2123r95. . The authors thank
Dr. G. Vasseur and an anonymous reviewer for
critical and useful review.
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