Coimbra-Tomar fault zone (Northern Portug

1
Late Cenozoic basin opening in relation to major strike-slip faulting along the PortoCoimbra-Tomar fault zone (Northern Portugal)
ALBERTO GOMES1, HELDER I. CHAMINÉ2,3, JOSÉ TEIXEIRA3, PAULO E.
FONSECA4, LUÍS C. GAMA PEREIRA5, ARY PINTO de JESUS6, AUGUSTO PÉREZ
ALBERTÍ7, MARIA A. ARAÚJO1, ALEXANDRA COELHO3, ANTÓNIO SOARES de
ANDRADE3 and FERNANDO T. ROCHA3
1
Departamento de Geografia, FLUP (GEDES), Universidade do Porto, Via Panorâmica
s/n, 4150-565 Porto, Portugal (E-mail: [email protected])
2
Departamento de Engenharia Geotécnica, Instituto Superior de Engenharia do Porto
ISEP, Porto, Portugal
3
Departamento de Geociências, Universidade de Aveiro (MIA), Aveiro, Portugal
4
Departamento de Geologia, Universidade de Lisboa (LATTEX), Lisboa, Portugal
5
Departamento de Ciências da Terra, Universidade de Coimbra (GMSG), Coimbra,
Portugal
6
Departamento de Geologia, Universidade do Porto (GIPEGO), Porto, Portugal
7
Departamento de Xeografía, Universidade de Santiago de Compostela, Spain
ABSTRACT
Northern Portugal is located in a tectonically complex area affected by major strike-slip
zones, namely the NNW-trending Porto-Coimbra-Tomar fault zone and the NNE-trending
Verin-Régua-Penacova sinistral strike-slip fault. Within this region, the sector between
Albergaria-a-Velha and Águeda is crucial since it is highly affected by large-scale strike
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slip faults and extensional deformation events. Late Cenozoic tectonics in Northern Iberia
resulted from the collision of the Africa and Eurasia plates especially in the eastern segment
of the Azores-Gibraltar plate boundary. The continued plate indentation originated the
movement of major strike-slip faults in the Iberian Massif. The movement on these faults,
accompanying the regional stress-field during the Early Miocene, initiated the formation of
incipient Cenozoic pull-apart basins.
This study examines the Albergaria-a-Velha−Águeda fault segment, in an attempt to clarify
the dynamic relationship between this active fault zone and the evolving landscape. Three
geomorphological sectors were identified in the Albergaria-a-Velha region: i) a littoral
platform consisting of a polygenic erosion surface overlain by Late Cenozoic alluvialfluvial sequences; ii) a tectonically controlled basin (Valongo do Vouga basin) located
between hillslopes of two river valleys and normal faults with N-S orientation, where Late
Cenozoic subsidence is suggested by an influx of alluvial sandy-conglomerates; and iii) a
domain of inner elevations of wide metapelitic landforms. Reactivation of the prevailing
NNW-striking Upper Proterozoic/Palaeozoic basement is a regionally important control on
the orientation and kinematics of Late Cenozoic faults. Thus, the opening and development
of these basins was influenced by the intersection of the NNW-trending dextral faults with
NNE-trending sinistral faults associated with north-south shorthening and east-west
extension.
Keywords Basement, Iberian Massif, Porto–Coimbra–Tomar fault zone, relief, strike-slip
basins.
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INTRODUCTION
The tectonosedimentary architecture, mechanical and seismic properties of large faultsystems can be better understood if structural geometries within the fault zone are
characterized (e.g., Sylvester, 1988; Woodcock and Schubert, 1994; Davison, 1994;
Richard et al., 1995; Stone et al., 1997; Lagarde et al., 2000; Davis, 2000; Friend et al.,
2000). In general, large-displacement faults produce wide deformation zones either by
extension or by compression. Wide damaged zones develop a complex internal geometry,
which may influence the intrinsic behaviour of major faults (Cunnigham et al., 2003).
These structures comprise upward-diverging faults, typically cutting antiformal push-ups or
synformal pull-aparts, that normally form an anastomosed network of faults, where strikeslip faulting is one of the most important deformation mechanisms (e.g., Cabral, 1989;
Woodcock & Schubert, 1994; Dooley & McClay, 1997; Rahe et al., 1998). Out-ofsequence thrust faults are commonly found in many orogenic belts (e.g., McKerrow et al.,
1977; Morley, 1988; Friend et al., 2000; Little & Mortimer, 2001; Gutiérrez-Alonso et al.,
2004). During geological times, however, fault systems frequently undergo modifications
of their pattern style in response to variations in their regional stress field (e.g., Andeweg &
Cloetingh, 2001; Ribeiro, 2002; Arjannikova et al., 2004).
The complexity of sedimentary basins associated with strike-slip fault systems are almost
as great as that observed for all other types of basins (e.g., Badham, 1982; Sylvester 1988;
Woodcook & Schubert, 1994; Wood et al., 1994; Richard et al., 1995; Wakabayashi et al.,
2004). Furthermore, strike-slip fault systems within continental crust are likely to
experience alternating periods of extension and compression as slip directions adjust along
major crustal faults (Crowell, 1974; Ingersoll, 1988). The occurrence of offsets and
bifurcations in strike-slip fault systems can lead to the formation of either transtensional or
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transpressional areas (Mann et al., 1983; Woodcook & Schubert, 1994; Basile & Brun,
1999; Wakabayashi et al., 2004). The shape of pull-apart varies with their progressive
development from spindle shape, through ‘lazy-S’ or ‘lazy-Z’ and rhomb shapes, to
complex multi-rhomb shapes (Mann et al., 1983).
The basement of intra and/or interplate settings consists mainly of exposures of highlydeformed crystalline rocks, often with a smooth topography (Cabral, 1989; Ribeiro et al.,
1996; Bonnet et al., 2000; Cunnigham et al., 2003). According to recent data from studies
on surface processes and topographic relief of the formation of basement rocks in a
collisional framework, the gravity collapse during the orogenic late stage of evolution is an
important mechanism by which the elevation of mountain chains is strongly reduced
(Summerfield, 2000; Burbank & Anderson, 2000).
Therefore basement relief cannot be regarded merely as the result of long-term erosional
activity (Stone et al., 1997; Bonnet et al., 2000). The study of the landforms and the
adjacent depocentres generated by active and persistent tectonic processes may,
consequently, provide insights into fault-generated mountain fronts and large-scale relief
development related to tectonic uplift of the basement (e.g., Silva et al., 1993, 2003;
Eusden et al., 2000; Bonnet et al., 2000; Tippet & Hovius, 2000). Recent studies of relief
development in crystalline basement relate mainly to the recognition and reconstruction of
old palaeosurfaces in relation to the geological record of ancient kinematic processes (e.g.,
Summerfield, 2000; Bonnet et al., 2000). Large-displacement faults often produce wide
zones of deformation that commonly have complex internal geometries, which in return
may lead to significant modifications of the properties of the discontinuities and the
sedimentary deposition.
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Tectonics drives geodynamic background processes that, over time, directly shape surface
topography. The effects of tectonics on topography occur over a large range of temporal
and spatial scales. Surface topography in active deformation zones also incorporates the
effects of processes such as climate, lithology and vegetation. The relationships between
tectonics and relief formed by drainage network development in active zones depend
directly on the role of erosion (displayed through isostatic responses and climate change)
on the control of large-scale tectonic uplift (Bonnet et al., 2000; Tippet & Hovius, 2000;
Schumm et al., 2000).
The Porto–Coimbra–Tomar (PCT) fault zone is an almost linear narrow belt with a NNW
trend comprised within the crystalline polymetamorphosed belt of the Iberian Variscides
(e.g., Lefort & Ribeiro, 1980; Ribeiro et al., 1990a, 1996). During pre-Mesozoic times
(Late Proterozoic to Palaeozoic; Beetsma, 1995; Chaminé et al., 2003b) this fault system
was a major dextral imbricated thrust zone (e.g., Gama Pereira, 1987; Dias & Ribeiro,
1993; Chaminé 2000). Moreover, the PCT fault zone (Fig. 1) is part of the Porto–Tomar–
Ferreira do Alentejo major shear zone (Chaminé, 2000; Ribeiro et al., 2003; Chaminé et al.,
2003a,b), and is enclosed within the Western Iberian Line (Chaminé et al., 2003a,b). WIL
delineates a NNW trending tectonic corridor more than 520 km long, from Tomar
(Portugal) to Finisterre (Galicia, Spain). This westernmost deformation corridor is
characterized by out-of-sequence thrusting with affinity to the Ossa-Morena Zone
(Chaminé et al., 2003a,b). Mainly it comprises dextral strike-slip parallel overthrusts and
dip-slip faults, as well as, normal faults originating from transtensional basins forming
within the inner part of the major shear zone. These configurations typically formed
releasing bend structures during the Variscan Orogeny. This scenario is most probably
responsible for the scattering of several imbricated metapelitic and blastomylonitic slices of
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Late Proterozoic/Palaeozoic tectonostratigraphic units (Chaminé et al., 2003a,b, in press,
2004).
The present study analyses the Late Cenozoic landscape development and the structural
evolution of a collapsed transpressive system along the Albergaria-a-Velha–Águeda
segment (Valongo do Vouga basin) of the PCT fault zone. The purpose of this assessment
is to achieve a better understanding of the geometry of near-surface strike- and dip-slip
structures and their relationships to the evolving surface landscape. The observations of the
internal structure of this large fault zone reported here provide new insights into the
geometry and kinematics of basin evolution during the geological record. This work
outlines, initially, the geotectonical and geomorphological settings of the PCT fault zone,
and then describes detailed field observations of Albergaria-a-Velha–Águeda fault segment
in an attempt to clarify the dynamic relationship between the principal displacement zone
and the evolving landscape.
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REGIONAL GEOLOGICAL SETTING
The PCT fault zone was formed as a large lineament of complex accretionary thrusts during
Variscan times. It comprises autochthonous and parautochthonous tectonostratigraphic
units (Fig. 2) of low- to high-grade metamorphic rocks, as well as allochthonous units of
medium- to high-grade metamorphic rocks, assumed to be of Upper Proterozoic age (e.g.,
Gama Pereira, 1987; Beetsma, 1995; Chaminé et al., 2003a,b; and references therein).
The general features for the region suggest two main regional tectonometamorphic stages
of Variscan deformation (e.g., Severo Gonçalves, 1974; Gama Pereira, 1987; Chaminé
2000) sometimes overprinting an earlier Cadomian migmatite sequence (e.g., Gama
Pereira, 1987; Dias & Ribeiro, 1993; Chaminé, 2000). The first Variscan stage produced
important folding and thrusts, as well as the dominant regional cleavage. The second
regional stage (related to Central-Iberian Zone Variscan-D3; Dias & Ribeiro, 1995), also
associated with mega-shear zones, produced a typical C-S shear deformation fabric and a
non-coplanar cleavage schistosity with mylonitic or blastomylonitic foliation and
crenulation. The metamorphic recrystallisation coincided with the first Variscan stage, and
continued in the second stage, when the major event of deformation resulted in
metamorphic blastesis and metasomatism (e.g., Severo Gonçalves, 1974; Gama Pereira,
1987; Chaminé, 2000). Two major fault branches of the S. João-de-Ver thin skin thrust
sheet (Chaminé, 2000), in a N–S direction, dominate the Albergaria-a-Velha–Águeda
sector. During the Late Cenozoic times a sinistral strike-slip faulting was associated with
transtensional kinematics triggered by the post-orogenic collapse of the structure along the
ancient Porto–Coimbra–Tomar thrust planes. These processes generated a multitude of
discrete ENE-WSW, NNE-SSW to NE-SW regional fault systems (e.g., Açores-Carvalhal
fault, Vouga River fault) with the generation of several pull-apart basins.
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Basement rocks of the Albergaria-a-Velha–Águeda sector generally comprise
subgreenschist to amphibolite grade metasedimentary rocks, as well as metavolcanic and
blastomylonitic rocks (Chaminé et al., 2003a). The Upper Proterozoic substratum rocks
(Beetsma, 1995) comprise monotonous greenschists, phyllites, slates and staurolite-garnet
schists, which generally dip NE and strike 25ºNW; these bed-rocks dominate the structure
of the Northern Águeda region. These metapelitic rocks are internally folded and foliated
reflecting at least two phases of Variscan regional ductile deformation. Cutting the
basement are various granitoids and blastomylonitic rocks of early-late Palaeozoic age
(Chaminé et al., 1998). Unconformably overlying the polymetamorphic basement and
infilling the pull-apart basins are Upper Devonian/Early Carboniferous fossiliferous black
shales (Chaminé et al., 2003b).
Triassic coarse clastic sediments (red conglomerate-sandstone deposits) are also found in
the region. The basement in the region is largely covered by post-Miocene continental
sedimentary deposits (e.g., Soares de Carvalho, 1946a; Palain, 1976; Telles Antunes et al.,
1979). The tectonostratigraphy of the Porto–Albergaria-a-Velha–Águeda sector is
summarized in Table 1.
METHODS
Structural and morphotectonic mapping of the Albergaria-a-Velha−Águeda segment on the
PCT fault zone, during Late Cenozoic times, was our first goal. Based upon fault
kinematics identified in the field and theoretically expected fault geometries, we attempted
to reconstruct the original structural system. The landforms were also mapped in order to
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integrate fault kinematics and landform generation over time; relief formation was then
used to help understand the development of the brittle fault system.
Structural geomorphology and landform maps were created by using a combination of airphoto interpretation, standard field mapping and digital terrain models. Portuguese Aerial
Mapping photos at 1/33.000 and 1/15.000 scales, obtained from the National Army
Geographical Institute, were used for photo-interpretation of tectonic network lineaments
and as base maps for compilation and fieldwork. Bedrock structural geology of the
Albergaria-a-Velha−Águeda region has been taken from the sketch geological map of
Águeda presented by Soares de Carvalho (1946a) and further updated, while for the
Albergaria-a-Velha sector, information was taken from Severo Gonçalves (1974) and
Chaminé (2000). Figure 3 presents the new geological map synthesis achieved for the
region under study after the fieldwork campaigns. This geological map was used as a basis
for further refinements in order to better understand the overall lithological and structural
framework.
Detailed information on the pre-Mesozoic evolution of the PCT basement rocks is beyond
the scope of this paper and the main results are published in Chaminé et al. (2003a,b, 2004,
in press) and Fernández et al. (2003).
MORPHOLOGY AND FAULT ARCHITECTURE
The studied segment (Albergaria-a-Velha–Águeda area, NW Portugal) consists of a 25km
long and a 5km wide asymmetric fault system, bounded to the South by Águeda River and
to the Northeast by Caima River, that narrows northwards to a single fault trace. In between
the bounding faults of the structure there are several subsidiary fault scarps that initially
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formed an imbricate set of footwall propagating thrust faults. Thus, inside corridors of the
PCT dextral strike-slip zone, major strain partitioning boundaries were defined (Dias &
Ribeiro, 1993). These strike-slip partitioning corridors separate predominantly preMesozoic regional-scale oblique strike-slip faults, from thrust-dominated structures
(Chaminé, 2000). The pattern of Late Cenozoic faulting indicates NNW-driven
compressional stresses from the Africa-Eurasian collision occurring in the eastern segment
of the Azores-Gibraltar plate boundary (Cabral, 1995; Ribeiro et al., 1996; Andeweg et al.,
1999; Cloetingh et al., 2002; Ribeiro, 2002). Consequently, Alpine stress trajectories of the
Atlantic margin clearly changed from the post-Miocene to the present-day (e.g., Cabral,
1989; Ribeiro et al., 1990b; Jabaloy et al., 2002; Cloetingh et al., 2002; Andeweg, 2002).
These features suggest a recent change of the regional strain regime by a reversal of
kinematics inducing tectonic activity on the NNE strike-slip faults. The recent geotectonic
evolution of Northwest Iberia has been dominated by NNE trending fault zones reactivating
inherited crustal structures in a sinistral strike-slip regime, namely Verin-Régua-Penacova
fault and Bragança-Vilariça-Manteigas fault (Cabral, 1989, 1995; Brum Ferreira, 1991;
Ribeiro, 2002). Nevertheless, total displacement as evidenced by regional geomorphic
framework is actually the result of the periodic reactivation of different PCT fault segments
since the end of the Variscan Orogeny until the present-day (Ribeiro et al., 1990b; Cabral,
1995).
The morphotectonic and geological surveys allowed us to produce an original outline map
of the crystalline basement relief (Fig. 3). This new structural geology and geomorphology
mapping of Northern Águeda region, demonstrates that the scale relief development is
strongly associated with the existence of scarps inherited from PCT principal displacement
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zone. In addition, the relief development in this segment was mainly controlled by Late
Cenozoic tectonic uplift during fault reactivation and interference.
The study area , the Valongo do Vouga tectonic basin, occupies a NNW-SSE narrow strip
along the PCT fault zone, which is limited by a major range bounding fault and an uplift
block. The regional block steps show two main orientations, NNW-SSE to N-S and NESW. The Albergaria-a-Velha–Águeda segment of the PCT fault zone is the most prominent
scarp of this morphology, with a length of 35km and a height that can reach 200m in some
localities.
Filling the tectonic basin are unconsolidated Miocene-Pliocene alluvial continental deposits
that are outlined as follows.
(i) The base of the sequence is composed by massive gravel-boulder conglomerate beds
with a grey-brown sandy matrix. The bed thickness ranges from 4 to 5m. The sorting is
very poor with clasts randomly oriented. The clasts have a maximum clast size ranging
from 40 to 70 cm and are dominated by white quartz (83% of the total) and quartzite; both
clast types are mainly sub-rounded. Individually, beds are matrix-supported conglomerates
lacking sedimentary structures, a characteristic feature of debris flow deposits.
(ii) In the middle of the sequence, one can find massive grey-white silty-muddy beds which
are interpreted as alluvial plain deposits.
(iii) The top of the sequence comprises massive coarse sandy beds with a red silty matrix.
Outcrops include some pebble or conglomerate lenses, with rounded clasts showing a red
patina (iron oxides). The clasts have been reworked, and are dominated by quartzite (ca.
64%), white quartz (33%) and metagreywacke (3%). In addition, within these beds there
are sets of trough cross-stratified sandstone facies that were interpreted as representing
stream channel deposits.
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The early Quaternary fluvial terraces are characterized by massive clast-supported coarse
sands with a grey-white silty matrix. The sorting is moderate to poor. The deposits include
some conglomerate lenses with elongated and imbricated clasts, which are also rounded.
The clasts have an maximum clast size of 20 cm and are composed of white quartz (37%),
slate (20%), metagreywacke (16%), granite (14%), gneiss (9%) and quartzite (4%). Metrescale sets of cross-stratified sandstone were identified in the stream channel deposits.
Palaeocurrent directions are towards N225ºE ± 10º and show a small amount of dispersion.
The fluvial terraces are covered by colluvium deposits, which consist of grey-brown very
fine silty-clay mud with some mineral grains (quartz, mica), estimated to be ca. 5m thick.
Three distinct morphostructural sectors were identified in this strike-slip fault segment (Fig.
4A): i) a littoral platform in Albergaria-a-Velha-Águeda region (western sector) subdivided
by a meridian tectonic corridor, ii) a tectonic basin in Valongo do Vouga area (central
sector), composed by two morphotectonic compartments, and iii) a domain of smooth
stepped elevations to the interior (northeastern sector). Their main morphotectonic
characteristics, described below, bear the signature of the major tectonic structures
occurring in the Albergaria-a-Velha-Águeda fault segment.
Albergaria-a-Velha littoral platform (western sector)
The Albergaria-a-Velha littoral platform corresponds to a planar surface gently dipping to
the west of Caima and Vouga river valleys (Soares de Carvalho, 1946a,b, 1949). The area
is dominantly composed of greenschist grade basement rocks. In the study sector, the
platform shows an elevation varying between 60 and 200 m (on its eastern side) and ends
against the S. João-de-Ver thrust sheet, which is marked by small elongated hills, of N-S
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orientation (Fig. 4), and extends from Fradelos to Senhora do Socorro (216 m). This
planation surface is interrupted by a meridian graben of flat-topped hills which are 20 m to
40 m, below the littoral platform top. According to Brum Ferreira (1980) the platform is a
polygenic erosion surface overlain by Late Cenozoic alluvial-fluvial sequences (Cunha et
al., 2005). The down-thrown hills are separated from the tectonic basin of Valongo do
Vouga and, particularly, from the PCT fault zone (s.str.), by deeply incised river valleys
which are under regional tectonic control (Fig. 4).
Valongo do Vouga tectonic basin (central sector)
Valongo do Vouga basin corresponds to a tectonic corridor located between the western
hillslopes of Caima and Vouga river valleys and the normal faults, of N-S orientation,
extending from the Telhadela to Águeda regions (Fig. 4). It is characterized by the presence
of steep hills to the west, whereas a more gentle topography is observed to the east (near
Albergaria-a-Velha platform). The Valongo do Vouga basin is composed of two major
morphotectonic compartments (Fig. 5): i) the Carvalhal compartment at north of Vouga
river and ii) the Soutelo compartment to the south.
The Carvalhal compartment is a major uplift block with a SE tilt due to NW-directed
thrusting along NW inner thrust fronts. Along this block, the main frontal fault scarp is
remarkably linear (Figs. 5 and 6). The main fault zone is made up of a distinct 5-8 m wide
zone of soft black-greenish gouge. This compartment is bounded, on the east, by the PCT
strike-slip fault with a normal component, on the west by the eastern branch of the S. Joãode-Ver thrust sheet and, on the south, by the Vouga River normal fault (Fig. 5). At this
location, the Vouga River normal fault is up to 4.5km long and contains many sub-parallel
fracture surfaces striking on average N45º ± 5ºE and dipping vertically or steeply to SE.
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The area is dominantly composed of metapelitic substratum rocks of greenschistamphibolite grade. Where the Vouga River crosses the Valongo do Vouga tectonic basin,
three levels of Quaternary fluvial terraces can be distinguished. In the vicinity of Carvoeiro
(Fig. 5), several terraces are preserved on the right bank of the valley. To the north of the
site, lies the highest terrace, made of unconsolidated alluvial deposits (conglomerates and
sands), 40 m above the present-day riverbed. A NE-SW normal fault scarp passing along
the Vouga valley is responsible for a vertical displacement of 8-10 m between the highest
fluvial terraces near Carvoeiro. The NNE-SSW sinistral component is clearly observed on
the Digital Terrain Model (DTM) where several rivers show deflection and a typical offset
corner in a sinistral sense (Fig. 4A). In the Carvalhal compartment the morphology is
characterized by the occurrence of deeply incised valleys that down-cut the relief,
localising steep sloped hill tops.
According to Soares de Carvalho (1946a), the Soutelo compartment is down-thrown with a
gentle SE tilt due to the NW-directed thrusting with rotation (Fig. 6). The Soutelo
depression shows a mean altitude of 24 m and is filled with Miocene to Quaternary
sediments. The mean thickness of these deposits is around 30 m, their origin being alluvial
or colluvial. The Early Triassic red sandstones outcrop mainly in the western part of the
compartment. Morphostructural highs, aligned in a N-S direction and reaching 104 m,
comprise metasedimentary rocks outcropping in the eastern part of the Soutelo block. The
linear trend of the scarp fault is indicative of the near-vertical dip angle of this segment
included in Albergaria-a-Velha-Águeda fault system (Figs. 5 and 6).
Inner elevations domain (northeastern sector)
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The inner elevations domain is a major uplift block positioned to the northeast of Valongo
do Vouga basin (Fig. 4). It is composed of several steep hills (reaching 600m of height).
The western front of this group of hills is marked by a major active fault system that is
clearly visible on aerial photographs and DTM. This fault is part of the regional PCT
system (Chaminé et al., 2003a) and in this study is referred to as the Albergaria-a-Velha–
Águeda fault. The presence of wide blocks of metapelitic rocks at high elevations, bounded
by deep valleys, more incised than those of Valongo do Vouga basin, is the most prominent
feature. Near the main fault, the relief is dominated by elongated N-S residual resistant
quartzitic ridges, with altitudes reaching 400 m. The inner elevations contrast with the
smooth surfaces to the west (200 m), and define chiefly regional tectonic lineaments of N-S
to NNW-SSE preferential trends.
A MORPHOTECTONIC MODEL: INTERPRETATION AND DISCUSSION
The data presented here for Valongo do Vouga basin provides new insights to the Porto–
Coimbra–Tomar major strike-slip fault zone. in addition to improving our knowledge on
the evolving landforms and deposits developed in this region, they clarify the
understanding of the geodynamic evolution of the Albergaria-a-Velha–Águeda fault
segment. The complex geometry originated from a stretched zone encompassed between
two tectonostratigraphic Iberian mega-domains (Ossa-Morena and Central-Iberian Zones)
with a long polyphase geotectonical history of activity (Gama Pereira, 1987; Chaminé,
2000; Chaminé et al., 2003a,b), explains the complex general pattern of the pre-Mesozoic
substratum, the so-called Porto-Albergaria-a-Velha Proterozoic metamorphic belt
(Chaminé, 2000).
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Given the geological and geomorphological complexity of Valongo do Vouga tectonic
basin we focused the present work on the following aspects: i) the occurrence of rigid
crustal anisotropy in mechanically weak pre-Mesozoic metasedimentary and
blastomylonitic basement rocks; ii) Late Cenozoic movements occurring along the NNE
trending fault segments displaying both a normal and a left-lateral component thus
indicating a transtensional regime; iii) the morphotectonic compartments (Carvalhal and
Soutelo blocks) of the Valongo do Vouga basin, which show major fault scarps with SE
tilting due to NW-directed and bounding the NW sides of uplifted/downthrown blocks. By
contrast, the fault architecture on other morphostructural sectors is essentially large westtilted thrust blocks.
Altogether, our data suggest that the morphology of Valongo do Vouga basin has been
formed by Late Cenozoic displacement on offset segments of the PCT fault system due to
sinistral movement of the NNE-trending faults (Fig. 7). Originally, Valongo do Vouga
basin had a flat-rhombic shape with its long axis oriented in a North direction, and bounded
by arc-shaped sidewall faults linking two offset segments of this major N-S trending fault
system. A sigmoid structure then succeeded as the surface expression of a fault-bounded
frame developed along the margin of the basin. The terraces that formed along the southern
basin sidewall fault system are interpreted as corresponding to down-faulted blocks. The
structure may be formed either before or simultaneously with NE-SW to E-W faults. The
basin sidewall faults change from curved in the middle to steep and planar toward the
corners of the basin where they are connected to the principal displacement zones of the
PCT fault system. The basin floor consists dominantly of pre-Permian metamorphic rocks
and Early Triassic sedimentary cover (Figs. 3 and 6), surrounded by sidewall fault scarps of
up to 200 m high. Likewise, stretched out small depressions are defined on the secondary
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valleys floor (e.g., Telhadela, Ribeira-de-Fráguas, Vale Maior, Mouquim and Soutelo
locations). The tendency for a local subsidence pattern in this corridor is marked by the
deposition and preservation of Miocene-Pliocene alluvial fans and the conservation of the
main Quaternary fluvial terraces of Vouga, Caima and Águeda rivers.
Another relevant aspect of the tectonic control on the drainage network is the emergence of
a lithologic resistance barrier (e.g., the Armorican quartzite) along a NE-SW regional fault
system as result of bedrock incision. In fact, the spatial organization of the regional
drainage network is strongly controlled by the large-scale topography, namely by the
location of high elevation domains of the Armorican quartzite relief. The main rivers (e.g.,
the Caima and Vouga Rivers) flow along the principal displacement zone, with a north to
south trend, before reaching the Atlantic Ocean. The river valleys have moderate depths,
ranging from a minimum of 150 to a maximum of 200 m.
CONCLUSIONS
The Porto–Albergaria-a-Velha–Águeda metamorphic belt, which is at least 65 km long, is
segmented into several morphotectonic compartments. One of these compartments is the
Albergaria-a-Velha–Águeda fault system, which reaches about 35 km long and is the focus
of this study. The major faults bounding the Valongo do Vouga basin are inherited ancient
discrete structures that have been reactivated in a transtensional tectonic regime since, at
least, Late Cenozoic times.
An evolutionary model for the Valongo do Vouga tectonic basin may thus be constructed,
using the comparative analysis of structural mapping and geomorphological data. A
tentative approach has been developed by linking the structural Variscan substratum
evolution of the Albergaria-a-Velha-Águeda segment of the PCT fault system to its
18
dynamic landscape elements in an active tectonic setting. Indeed, the basement structure
with uplifted and deformed Triassic deposits, caused by post-Variscan kinematics, in the
southwestern part of Valongo do Vouga region, defines the late evolution of the segmented
fault system. The formation of the early Northern segment consisted of
uplifted/downthrown fault blocks and brittly deformed, due to Late Cenozoic displacement
along the Albergaria-a-Velha–Águeda fault system and sinistral NNE-trending related
system (e.g., Verin-Régua-Penacova fault). Furthermore, these events were closely
followed by the deposition and displacement of the sedimentary succession on the faultblock. Finally, the formation of the present rhomboidal basin architecture may have
triggered the emergence of the NNE-SSW to NE-SW conjugated trending faults.
ACKNOWLEDGEMENTS
We gratefully acknowledge Peter Friend (Cambridge), Gaspar Soares de Carvalho (Braga),
Christian Palain (Nancy) and João Cabral (Lisbon) for stimulating discussions and for their
assistance in fieldwork. This research was partially supported by TBA (FCTPOCTI/CTA/38659/2001), Geodyn (POCTI-ISFL-5-32/Lisbon), GROUNDURBAN
(POCTI/CTE-GIN/59081/2004) and FCT-SFRH/BPD/3641/2000 (Aveiro) research grants.
We acknowledge James Howard, Tim Dooley and Gary Nichols for constructive reviews
that helped to improve the clarity of the manuscript.
19
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27
FIGURE CAPTIONS
Figure 1 – Geotectonical setting of Iberian Massif with the location of the Porto-CoimbraTomar fault zone (adapted from Ribeiro et al., 1990a).
Figure 2 – Regional geotectonic framework of the Porto-Albergaria-a-Velha metamorphic
belt, Ossa-Morena Zone, W Portugal (adapted from Chaminé, 2000).
Figure 3 – Geological map of the Albergaria-a-Velha−Águeda region (NW Portugal).
Figure 4 – Morphotectonic map of the Albergaria-a-Velha−Águeda region. A)
Morphostructural sectors: Albergaria-a-Velha−Águeda littoral plataform (I), Valongo do
Vouga tectonic basin (II), Inner elevations domain (III); B) Digital elevation model of the
studied area, obtained from digitization of elevation contour lines of the 1:25.000 map scale
and generated by kriging. Ground resolution is 50 m. Shadowed relief map of the digital
terrain model, artificially illuminated from the West; C) Morphotectonical interpretation.
Figura 5 –(a) Morphostructural interpretation of the Valongo do Vouga tectonic basin; (b)
view of the Soutelo flat floor depression and the frontal scarp that ends her to west; (c)
Detailed view of a normal fault affecting Mio-Pliocene debris-flow deposits in the Soutelo
compartment in the east; (d): Normal fault of N-S orientation affecting Triassic sandstones
near Águeda; (e): view west of old brittle frontal thrust zone (PCT fault zone) cutting
28
Upper Palaeozoic black shales bearing metasomatic carbonates, near Soutelo; (f): Normal
fault of NW-SE direction affecting Triassic sandstones near Vouga river).
Figure 6 – Cross-sections through Valongo do Vouga tectonic basin. Cross-sections are
constructed perpendicular to Porto−Albergaria-a-Velha−Águeda fault system (see figure 3
for location and explanation).
Figure 7 – A tentative 3D model block diagrams (a and b) and structural sketch map (c)
illustrating the relief development related to pre-Mesozoic substratum legacy of the Late
Cenozoic infill basins. a) Early basin formation fault-block and growth fault sequence
development due to dextral displacement along the Porto−Albergaria-a-Velha fault system;
b) illustration of the present rhomboidal basin architecture (PDZ: principal displacement
zone). Shear zones and terranes: Western Iberian Line (WIL), PTFASZ: Porto–Tomar–
Ferreira do Alentejo dextral major shear zone, FFT: Ferreira do Alentejo–Ficalho thrust,
TCSZ: Tomar–Córdoba sinistral shear zone, FST: Farilhões suspect terrane, VRPF: VerinRégua-Penacova Fault, BVMF: Bragança-Vilariça-Manteigas Fault.
29
Table 1 – Summary of stratigraphic and tectonometamorphic features of the Porto–
Albergaria-a-Velha fault system.
Stratigraphy
Tectonometamorphic events
Timing
(Chaminé et al ., 2003a,b)
Orogeny
Porto–Albergaria-a-Velha-Águeda platform
¾
Sedimentary Cover
Diagenesis, rifting process, tectonic inversion
Early Triassic / Quaternary
Alpine
Metasedimentary basement
Ossa-Morena Zone
Allochthonous
Black shales bearing metacarbonates
Early Carboniferous (Namurian),
Very low-grade metamorphism, organic-rich Upper Devonian
(Givetian/Frasnian)
rocks
Late Variscan
Parautochthonous/Autochthonous
Micaschists, garnetiferous quartzites, phyllites;
migmatites, gneisses
Middle- to high-grade metamorphism, folding
in higher-grade areas, thrusting
Early deformation, low- to high-grade
metamorphism, peak metamorphism (ca. 311
Ma), folding in higher-grade areas; Post- Cambrian [?] / Upper Proterozoic
metamorphism deformation, cross-folding,
shear zones, thrusting, extensional cleavage
Pre- and late-Variscan;
Cadomian [?]
Central-Iberian Zone
Parautochthonous/Autochthonous
Armorican quartzites, fossiliferous gray slates
Ordovician
Low-grade metamorphism (greenschist facies),
folding, variscan structures pre- to syn-peak
metamorphism, shear zones, fabric development
Schists, greywackes
Variscan and Cadomian [?]
Upper Proterozoic
Granitic rocks
¾ Lavadores granite
Post-tectonic granite
298±12 Ma
¾ Oliveira de Azeméis–Feira–
Lourosela granitic belt
Granitic antiform structure
320±3 Ma; 379±12 Ma; 421±4
Ma; 419±4 Ma
¾ Ossela–Milheirós de Poiares
blastomylonitic belt
Granitic synform structure
¾ Foz do Douro Complex
Shear zones, mylonitic fabric
Late-Variscan
Variscan and Pre-Variscan
575+5 Ma; 607±17 Ma
Cadomian