ELSEVIER
Precambrian Research 77 (1996) 1-15
Structural style of basin inversion at mid-crustal levels: two
transects in the internal zone of the Brasiliano Ara uai Belt,
Minas Gerais, Brazil
W. Dickson Cunningham a'*, Stephen Marshak a, Fernando F. Alkmim b
~'Department of Geology, University of Illinois, 245 Natural History Building, 1301 West Green Street, Urbana, IL 61801, USA
hDepartmento de Geologia, Universidade Federal de Ouro Preto, Morro do Cruzeiro, Ouro Preto, M.G. 35.400 Brazil
Received 10 January 1995; revised version accepted 18 July 1995
Abstract
The Araguai belt is the orogenic belt that directly borders the eastern margin of the Silo Francisco Craton m eastern Brazil.
Detailed structural investigations in the Governador Valadares region of Minas Gerais indicate that the amphibolite-to granulitegrade internal zones of the Ara9uai belt contain several major, west-vergent, crystalline overthrust sheets. These thrust sheets
contain approximately homoclinal east-dipping gneissic banding and are separated from one another by zones of isoclinally and
sheath-folded, ductiley sheared, metasedimentary units that behaved as mechanically weak glide horizons during deformation.
We interpret this regionally imbricated sequence of basement and cover to be the mid-crustal level manifestation of closure of a
mid-Neoproterozoic rift basin that existed to the east of the Sao Francisco Craton. The major thrusts, which are all cratonvergent, are of Brasiliano/Pan-African age (650--450 Ma) because they cut the Neoproterozoic Galil6ia batholith. Older fabrics
are locally preserved in the basement slices, and these fabrics may be relicts of the Transamazonian orogeny (2.0 Ga). Discrete
zones of ductile-brittle extension that were identified in several localities in the study area suggest the occurrence of postorogenic collapse following Brasiliano overthrusting, Alternations of rigid crystalline thrust sheets and highly deformed metasedimentary sequences, such as those of the Governador Valadares region, may be a common structural geometry at a depth of
15-20 km in modern regions of collision and basin closure.
1. Introduction
The Sao Francisco Craton is an Archean cratonic
block in eastern Brazil that is surrounded by cratonvergent Neoproterozoic ( B rasiliano) mobile belts. The
Brasiliano belts that surround the S~o Francisco Craton
have been the subject of limited detailed structural
investigation relative to Proterozoic orogens on other
continents. Consequently, questions remain regarding
*Corresponding author, now at: Department of Geology, University
of Leicester, University Road, Leicester, LEt 7RH, UK. Tel: 533
523649, Fax: 533 523918, e.mail: [email protected]
0301-9268 / 96 / $15.00 © 1996 Elsevier Science B.V. All rights reserved
SSD10301-9268 ( 9 5 ) 00039-9
how the S~o Francisco Craton amalgamated with other
Archean and Proterozoic blocks to form Gondwana in
Neoproterozoic time. As part of an ongoing project to
study the S~o Francisco Craton and its margins, we
carried out detailed structural studies along two approximately east-west cross-strike transects and at other key
outcrops in the Governador Valadares region of central-eastern Minas Gerais state (Fig. 1 ) in order to
characterize the basic structural framework of the area
and constrain its tectonic evolution.
The Governador Valadares region is located within
the northern Mantiqueira Province (Fig. 1 ), the coastal
2
W.D. Cunningham et al./Precambrian Research 77 (1996) 1-15
Espinhaqo. A tectonic study of the northern sector of
the Araguai belt is presented in Pedrosa-Soares et al.
(1992).
The northern Mantiqueira Province including the
Ara~uai and Atlantic belts is generally interpreted to
have been connected to the West Congo fold belt in
western Africa prior to the Early Cretaceous rifting of
the South Atlantic (Fig. 2; Schermerhorn, 1981;
Porada, 1989; Maurin, 1993). Previous interpretations
suggest that the Ara~uai belt is an inverted and overthrusted Mesoproterozoic ensialic rift basin that was
filled with Mesoproterozoic strata of the Espinha~o
Supergroup (Torquato and Foga~a, 1979; Marshak and
Alkmim, 1988; Machado et al., 1989; Porada, 1989;
Fig. 1. Location map of study area in eastern Brazil.
Proterozoic deformation belt between 32°S and 15°N
(Almeida, 1977, 1981; Almeida and Hasui, 1984).
Although there is some confusion in the literature
regarding geological and geographical subdivisions of
this region, the northern Mantiqueira Province is generally divided into a western zone--the Ara~uai belt
(Almeida and Hasui, 1984)--and an eastern zone-the Coastal Mobile belt (Mascarenhas, 1979) or Atlantic belt (Leonardos and Fyfe, 1974). The Ara~uai belt
includes the Serra do Espinhaqo fold and thrust belt, a
foreland fold and thrust belt (Fig. 1 ) formed during the
Brasiliano orogeny (650--450 Ma; Pflug, 1965; Cordani et al., 1980; Herrgesell and Pflug, 1986; Marshak
and Alkmim, 1988) and internal metamorphic belts to
the east. The Atlantic belt includes high-grade metamorphic rocks between the Rio Doce Valley and the
Atlantic coast. The boundary between the eastern and
western zones is poorly defined and may not be geologically significant. The area described in this report
is located east of the Serra do Espinhaqo and crosses
the Rio Doce Valley and thus overlaps both the Ara~uai
and Atlantic belts (Figs. 1-3).
The structural geology of the region east of the Serra
do Espinhaqo has not been studied in detail and most
previous geological investigations have focused on the
economic potential or metamorphic petrology of the
region (Souza, 1982; MOiler et al., 1986). Da Silva
and Ferrari (1976), Ferrari (1984) and da Silva et al.
(1987) have provided the most detailed accounts of
the general geology of the region east of the Serra do
undifferentiatedc o v e r rocks
~
Upper Proterozoiedeposits
~ r ' ~ basementaffected by Brasiliano/Pan Afric.
~
undifferentiatedrocksof the cratonsincluding
Archeanbasementand Proterozoiccoverrocks
Fig. 2. WestAfrica--easternBrazilGondwanareconstructionshowing locationof Archeancratons and Brasiliano/Pan-Africanbelts.
Adapted from Porada, (1989). Location of Fig. 3 is shown.
AS = AngolaShield;CC = CongoCraton;SFC = S~oFranciscoCraton; W C O = W e s t Congo Orogenic Belt; R D J = R i o de Janeiro;
LU= Lusaka.Thicklines are faults;barbs on upperplates.
W.D. Cunningham et al. / Precambrian Research 77 (1996) 1-15
S611ner et al., 1991; Dossin et al., 1993) and Neoproterozoic strata of the Sao Francisco Supergroup (Inda
et al., 1984; Babinski et al., 1993). However, the structural link between the Ara~uai/Atlantic belt and the
West Congo belt and their tectonic evolution are not
well understood partly because ( 1) the Ara~uai/Atlantic belts have received limited prior study, because (2)
significant parts of these belts are covered by Cretaceous or younger rift-related deposits along the South
American and African continental margins, respectively, and because (3) Brasiliano tectonism has substantially obscured an older structural history resulting
from the Transamazonian (2.0 Ga) orogeny.
The objectives of this study were to document for
the first time the detailed geometry and style of contractional deformation in the metamorphic internal
I
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3
zones of the Araquai belt east of the Serra do Espinhaqo,
to try to identify pre-Brasiliano structures if they exist,
to document along-strike changes in the Araquai belt,
and to better understand the processes of basin inversion that occur at a 15-20 km depth.
2. Geographic setting of study area
The region that we studied in this project is a hilly
area that covers approximately 400 km ~ in the vicinity
of Governador Valadares, a city of about 300,000 people that borders the 50-km-wide Rio Doce Valley, the
principal drainage system in the region (Fig. 1). Topographic relief in this region generally ranges between
100 m and 400 m. Hillsides are mostly grass- or scrub-
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gneisses
~
spinha9oSupergroup metasedimentary rocks
Dominantly chamockitic rocks
~S~o
Francisco Craton
platform sediments
0
50
kilometers
Fig. 3. Simplified geological map of part of the Ara~uai belt east of the Silo Francisco Craton showing studied transects. Asterisk marks location
of Fig. 12. IP = Ipatinga; DC = Dom Cavati; SO = Sobralia; GA = Galil6ia: GV= Governador Valadares; CT= Conceiq5o de Tronqueiras.
4
W.D. Cunningham et al./Precambrian Research 77 (1996) 1-15
covered, with local pockets of remnant forest. Because
of deep weathering, the best continuous exposures
occur along roadcuts or stream cuts. Thus, this report
focuses on two detailed structural transects that were
made along existing roads. The northern and longer
transect starts at the town of Galil6ia and continues to
Governador Valadares and Conceigao de Tronqueiras,
and the southern transect runs between the towns of
Dora Cavati and Ipatinga (Figs. 1 and 3).
3. Geology of the northern transect
3. I. Lithologic assemblages
The eastern end of the northern transect (Figs. 3 and
4) starts in the Galil6ia batholith, a Brasiliano intrusive
complex consisting of tonalites, granodiorites and diorites (Barbosa, 1964; da Silva et al., 1987). Plutonic
rocks of the batholith are locally covered by intensely
deformed roof pendants or inclusions of metasedimen-
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Fig. 4. Structural c r o s s - s e c t i o n o f n o r t h e r n transect f r o m t o w n s o f G a l i l 6 i a to Conceiqfio de T r o n q u e i r a s , Structural d o m a i n s are referred to in
text.
W.D. Cunningham et al. / Precambrian Research 77 (1996) 1-15
5
Fig. 5. Close-upview of thrust zoneat the westernmarginof the Galil6iabatholith.Viewis lookingsouth.
tary units including quartzites, quartz-biotite-garnetsillimanite schists and calc-silicate rocks that are correlated with the S~o Tom6 metasedimentary assemblage that crops out further to the east (da Silva et al.,
1987). A Rb-Sr isochron for a diorite pluton within
the batholith gives a 650 Ma crystallization age,
whereas seven K-Ar ages on biotite from different
intrusive phases within the complex give dates between
450 and 580 Ma (da Silva et al., 1987).
West of the Galil6ia batholith, bedrock consists of
three separate blocks of homoclinally dipping crystalline gneisses that are separated from each other by three
separate sequences of highly folded metasedimentary
units. The easternmost gneiss sequence, that occurs
west of the Galil6ia batholith and under the city of
Governador Valadares, is the Piedade Gneiss (Figs. 3
and 4; Coutinho, 1968; da Silva et al., 1987), which
contains a variety of upper amphibolite to granulite
grade, banded gneisses of felsic and mafic composition.
The age of the Piedade Gneiss is not well constrained.
A Rb-Sr isochron by da Silva et al. (1987) yielded a
560_+ 15 Ma age, whereas Teixeira (1982) obtained
K-Ar dates between 525-670 Ma and a Rb-Sr isochron
age of 2.2 Ga.
A second belt of high-grade biotite gneisses occurs
between Governador Valadares and Conceiq~o de
Tronqueiras (Fig. 4). Due to poor exposure, it is difficult to correlate this unit with other sequences. The
gneisses are lithologically heterogeneous, finely layered and contain garnet and locally muscovite. They
lack igneous textures and appear paragneissic.
At the west end of the transect, near the town of
Conceiq~o de Tronqueiras is a wide belt of heterogeneous biotite gneisses that can be correlated along strike
to the SSW with the Mantiqueira Gneiss complex
(Figs. 3 and 4; Trouw et al., 1986; Brandalise, 1991).
The Mantiqueira Gneiss complex consists of a variety
of upper amphibolite-grade felsic and mafic, biotiteand amphibole-rich gneisses generally interpreted to be
of igneous origin (Brandalise, 1991). Locally, the
gneisses are migmatitic. Rb-Sr ages for the Mantiqueira Gneiss complex from two different locations
give ages of 2160 Ma and 2760 Ma (Brandalise, 1991 ).
Similar gneisses occurring to the west and northwest
called the Guanh~es complex have yielded ages
older than 1900 Ma (Siga et al., 1982; Teixeira et al.,
1990) and may also be correlative to the Mantiqueira
Gneiss.
6
W.D. Cunningham et al./Precambrian Research 77 (1996) 1-15
Contoured Poles to Foliation
and Elongation Lineations
iiiil, i,
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Southern Transec
Poles to Foliation
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Fig. 6. Equal-area, lower-hemisphere stereoplots of structural data from both transects.
Separating the basement gneiss units from each other
and from the Galil6ia batholith are three separate zones
of metasedimentary units. West of the Galil6ia batholith and east of the Piedade Gneiss is a highly deformed
sequence of garnet-biotite schist, quartzite, quartzmuscovite schist and quartzo--feldspathic biotite
gneiss. These rocks are internally isoclinally folded and
highly sheared, and probably correlate with the Sao
Tom6 metasedimentary sequence that crops out southeast of Galil6ia and occurs as roof pendants within the
Galil6ia batholith. S~o Tom6 metasedimentary rocks
have not been dated, but must be older than the 650 Ma
dated Galil6ia intrusive complex that cuts it. The S~o
Tom6 sequence may correlate with Neoproterozoic
Espinhaqo Supergroup metasediments that crop out to
the north and west of the Governador Valadares region
(da Silva, 1987) or Neoproterozoic Macatibas Group
metasediments of the S~o Francisco Supergroup that
occur to the north (Pedrosa-Soares et al., 1992).
The Piedade Gneiss is bounded to the west by a 10kin-wide zone of highly sheared metasedimentary units
that are internally isoclinally folded and sheath-folded.
This metasedimentary sequence contains quartzites,
quartz-muscovite schists, biotite-garnet schists and
talc schists and is interpreted to be the along-strike
continuation of the Dora Silverio metasedimentary
group that occurs 120 km to the southwest (Brandalise,
1991). This correlation is based on lithological similarities between the two groups, analysis of map patterns and reconnaissance investigations that suggest
that the Dom Silverio Group crops out continuously
from the town of Dora Silverio to Governador Valadares (Radambrasil, 1983).
A third sequence of pelitic schists and quartzites
occurs immediately east of the Mantiqueira Gneiss
complex. Correlation of this sequence to known units
is not certain. It may be a structural repetition of the
Dom Silverio sequence.
3.2. Structural geology
For purposes of structural description, the northern
transect can be subdivided into seven different domains
W.D. Cunningham et al. / Precambrian Research 77 (1996) 1-15
7
Fig. 7. View looking north of west-vergent isoclinal fold from S~o Vitor domain metasedimentary units in northern transect. Width of photo is
5m.
(Fig. 4). These domains correlate closely with lithologic boundaries.
Galil~ia domain. The majority of the Galilria batholith that was investigated in this study is isotropic and
undeformed. However, near its western boundary,
there are discrete zones of weakly developed, shallow
to moderately ESE-dipping foliation that are defined
by the parallel alignment of biotite grains and flattened
and aligned mafic xenoliths that are oriented parallel to
the biotite foliation. Linear arrays of biotite aggregates
locally form a visible lineation that plunges moderately
to shallowly east-southeast. Several generations of
tourmaline-rich pegmatite dikes occur within the batholith, and some are rotated in a top-to-the-west sense,
whereas others are unrotated and dip moderately to the
west-southwest. Other dikes occur as large tension
gashes that are locally asymmetrically boudinaged
indicating top-to-the-west shear sense.
Large inclusions and roof pendants ofmetasedimentary lithologies are common in the western zones of the
Galilria batholith. These units are everywhere intensely
folded into tight or isoclinal forms that verge WNW
with axial surfaces dipping between 30-00 ° ESE. Some
isolated inclusions of metasedimentary units with fold
axes trending in E-W directions may have been passively rotated during intrusion of the batholith or they
may indicate complex fold geometries prior to intrusion. Some fold forms are sheath-like and disharmonic
reflecting the high-strain conditions and competence
contrasts that existed between layers during deformation that preceded intrusion.
S~o Vitor domain. This domain is bounded to the
east by the western contact of the Galilria batholith and
to the west by the Piedade Gneiss. The western contact
of the Galilria batholith is a 5-kin-wide zone of ductile
top-to-the-west overthrusting that places the Galilria
batholith over mica-garnet schists and quartzites. A
progressive increase in foliation intensity, lineation
development, grain diminution and biotite content was
observed in the batholithic rocks when approaching the
contact from the east. Specifically in the zone of highest
strain, the granitoid has evolved into a schistose tectonite with mylonitic S-C fabrics, asymmetric tails on
feldspar porphyroclasts, asymmetric pegmatite or
quartz vein boudins and extensional crenulation cleavages. All kinematic indicators in the zone indicate top-
8
W.D. Cunninghamet al./Precambrian Research 77 (1996) 1-15
Fig. 8. Post-contractional,high-angle,extensionalshear zonefrom
SiloVitordomain,northerntransect. Viewis to northeast,east-sidedown shearsense. Fieldnotebookis 9.5 cm. across.
to-the-west thrusting shear sense (Fig. 5). Over the 5
km width of this shear zone, fabric consistently dips
between 10 and 35°E and biotite and quartz stretching
lineations consistently plunge gently to the ESE. The
geometrical relationship between foliation strike and
stretching lineation trends throughout the zone suggests
a minor component of sinistral displacement accompanied west-directed overthrusting (Fig. 6). A few discrete, brittle, gouge-filled fault zones were found within
the overall ductile shear zone suggesting that contraction continued under brittle conditions. Within the
westernmost parts of the overthrust zone, pegmatites
and granite sheets that intruded approximately parallel
to the country rock foliation constitute up to 50% of
the rock volume. The rock locally has a lit-par-lit
appearance and the melt zones may have accommodated displacement prior to their crystallization (Hollister and Crawford, 1986). Within the overthrust
metasedimentary units, layer-parallel isoclinal folds are
common. These folds are cut by the granite sheets and
thus predate their emplacement.
West of the batholith overthrust zone is an 8.5-kmwide belt containing a series of west-directed overthrusts and pervasively sheared and folded
metasedimentary units (Fig. 7). Quartz-muscovitegarnet-sillimanite schists internally contain asymmetric fabric relationships, sigmoidal tails on garnets and
asymmetric boudins indicative of top-to-the-west overthrusting. Distinct west-vergent thrust faults that truncate foliation were identified in several areas, and these
faults are also locally folded. Some thrust faults have
gouge zones along brittle fault surfaces with down-dip
slickensides. In the S~o Vitor domain all folds, at all
scales, are asymmetric west-vergent tight to isoclinal
forms that are generally overturned and locally recumbent. Thus, the entire 8.5-km-wide zone records a history of intense west-vergent contractional strain.
At the western end of the S~o Vitor domain two
zones of northeast-striking southeast-dipping brittleductile normal faulting were documented (Fig. 8).
These zones truncate the contractional fabrics in the
wall rock and contain down-dip slickensides and asymmetric fabric relationships indicating east-side-down
normal displacement.
Piedade Gneiss domain. The Piedade Gneiss domain
is characterized by shallow easterly dipping gneissic
banding that is approximately homoclinal over a 15 km
width. On a regional scale, the Piedade Gneiss appears
to be a huge, relatively rigid, basement slab that dips
gently to the east. The consistent dip is visible from
large distances in outcrops along the Rio Doce and in
the high peak of Ibituruna and neighboring hills south
of the city of Governador Valadares. The unit is characterized by a strong gneissic banding and rarely a
biotite lineation. Folds with wavelengths larger than
0.5 m were not identified. Significantly, relict cm-scale
isoclinal folds and intrafolial fold hinges are locally
preserved indicating that an older fabric and fold-producing event preceded the development of the pervasive gneissic layering. Layer-parallel leucocratic
sheets, pegmatites and amphibolites are common constituents of the Piedade Gneiss and often are boudinaged or display pinch-and-swell geometry. Some
boudins are asymmetric indicating top-to-the-west
thrusting shear sense (Fig. 9). Locally, top-to-the-west
mylonitic shear zones occur within discrete zones.
Dora Silverio Group domain. This domain, in contrast with the bordering Piedade Gneiss domain, is
intensely folded internally and contains evidence for a
W.D. Cunninghamet al. / Precambrian Research 77 (1996) 1-15
9
Fig. 9. Asymmetricpegmatiteboudin from Piedade Gneissexposed in quarry in GovernadorValadares. View is to north, top-to-westshear
sense.
strike-slip component of displacement. WNW-vergent
tight to isoclinal folds are commonplace (Figs. 4, 10)
and sheath forms were also identified. The contact with
the Piedade Gneiss domain to the east is not exposed
but is interpreted to be a thrust fault emplacing the rigid
Piedade Gneiss block above the less competent and
internally isoclinally folded Dom Silverio Group metasediments. Several discrete thrust faults were identified
that separate or truncate units within the metasedimentary sequence. Schist units of the sequence contain
asymmetric fabric geometries, asymmetric tails on garnet porphyroblasts, and asymmetrically boudinaged
quartz veins that record top-to-the-west thrusting.
Three separate locations were identified that contain
evidence tbr approximately north-south strike-slip displacements. One zone is an oblique dextral thrust zone
whereas the other two zones have strike-parallel elongation lineations. One of these zones is near the Rio
Doce southwest of Governador Valadares where the
river flows linearly in a northeast direction for 20 km.
It is possible that the river valley is structurally controlled by a hidden strike-slip or oblique-slip fault.
Itatinga domain. This small, poorly exposed, domain
is distinguishable from adjacent metasedimentary
domains by an apparent lack of folding and by generally
homoclinal east-dipping foliation. The massive biotite
gneisses that comprise this domain locally contain
coarse meta-igneous textures.
Boa Esperanca Fold domain. This domain is a 3km-wide belt of folded and thrusted metasedimentary
rock. The quartz-muscovite-garnet schists and paragneisses that comprise it are folded into tight to isoclinal
west-vergent folds. The metasediments are internally
sheared and contain sigmoidal fabric geometries and
asymmetric boudinaged pegmatites and quartz veins
that indicate top-to-the-west thrusting. In one location,
discrete brittle thrust faults cut through a tbld limb
suggesting that contraction continued under brittle conditions. Numerous, small-scale, asymmetric folds
probably are parasitic to larger-scale W-vergent fold
structures that are not visible due to outcrop scarcity.
An east-vergent backthrust/fold pair was identified in
one location.
Mantiqueira Gneiss domain. This domain is similar
in appearance to the Piedade Gneiss domain because
10
W.D. Cunningham et al. / Precambrian Research 77 (1996) 1-15
Fig. 10. Intenselyfolded and attenuated metasedimentarylithologies from Dom SilverioGroup domain, northern transect. View is to south,
westerly fold-vergence.
of its regional E-dipping homoclinal gneissosity.
Locally, the Mantiqueira Gneiss contains small-scale
isoclinal and intrafolial folds which differ in geometry
and style from the consistent west-verging folds of the
other domains. We interpret these structures to represent an older fabric because we observed that it is
locally transposed into a gneissic foliation that parallels
the dominant fabric of the region. The resulting foliation is well developed and is defined by alignment of
biotite. It dips gently between 05--40° ENE. A weak
NE-trending biotite lineation was locally observed.
Several generations of pegmatite veins and large pegmatite stock-like bodies cut the sequence and are variably deformed to undeformed. The gneisses appear to
have behaved as a rigid block beneath the highly folded
metasedimentary units to the east.
4. Geology of the southern transect
4.1. Lithologic assemblages
The southern transect (Fig. 11) contains upper
amphibolite-granulite-grade garnet gneisses, massive
charnockite and quartz-rich metasedimentary rock. The
charnockite and garnet gneiss are correlated alongstrike with the Juiz de Fora basement complex that has
been studied further to the south, a unit that consists of
E)om
Ipatinga
Cavati
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Fig. l l. Structural cross-section of southern transect from towns of Don Cavati to lpatinga.
"~ II
W.D. Cunninghamet al. /Precambrian Research 77 (1996) 1-15
charnockite gneiss, enderbite, ortho-amphibolite, biotite-amphibole gneiss, garnet-biotite gneiss, tonalitic
and granitic gneiss and banded migmatite (Hasui and
Oliveira, 1984; Schulz-Kuhnt et al., 1990; Brandalise,
1991 ). Initial Sr ratios and Rb-Sr isochrons from Cordani et al. (1973) and da Silva et al. (t987) and PbPb ages from S611ner et al. ( 1991 ) indicate that the Juiz
de Fora complex is Archean. Younger Rb-Sr, U-Pb
and K-Ar dates, that correlate with Transamazonian
and Brasiliano events, were obtained by Delhal et al.
(1969), Cordani and Siga (1980), Bayer (1987) and
da Silva et al. (1987).
The metasedimentary sequences of the southern
transect include quartzite, quartz-mica schist and paragneiss. These sequences separate gneissic basement
slices from one another and are probably correlative
with similar lithologies described by Schulz-Kuhnt et
al. (1990) from a region near Rio Casca 75 km south
of our study area (Fig. 1). These units do not have a
lbrmation name and have not been dated.
4.2. Structural geology
The slices of charnockite that occur along the southern domain contain fabrics and deformation features
that are distinctly different from those found in noncharnockite lithologies. Thus, we describe structures of
the charnockite exposures and those of other lithologies
separately.
4.2.1. Structure o f charnockite exposures
Zones of charnockite crop out in four separate blocks
along the transect (Fig. 11 ). These rocks are massive
and coarse-grained, and are unfoliated to weakly foliated. Foliation in the charnockite, where it occurs, is
defined by the alternation of black biotite- and amphibole/pyroxene-rich layers with feldspar-rich layers.
This foliation is consistently vertical to steeply eastdipping. The foliation locally has a wavy appearance,
but mappable folds were not observed in outcrop. Mafic
layers, pods, and irregular angular fragments occur
locally and are aligned parallel to the foliation. Due to
its lack of foliation, much of the charnockite along the
southern transect appears to have retained its igneous
texture so the local foliation that we observed may be
a primary feature. The contrast in foliation intensity
between the charnockite units and adjacent units suggests that charnockite slices behaved as massive rigid
11
blocks during regional deformation. Shear zones and
elongation lineations were not observed in the charnockites, and the borders of the blocks were not
observed due to lack of exposure.
We suggest that the charnockite slices are bounded
by thrust faults on their western contacts because of
apparent repetition of lithologic sequences, because
they are bounded by thrust faults elsewhere in the
region (Almeida, 1981; Jordt-Evangelista and Mfiller,
1986), and because adjacent metasedimentary units
that are structurally below the charnockites are
intensely strained.
4.2.2. Structure o f non-charnockite exposures
In addition to charnockite, exposures in the southern
transect consist of paragneiss, other metasedimentary
units and orthogneisses that are structurally distinguished from the charnockite domains because they
contain a strong metamorphic foliation that is intensely
folded and/or sheared. At the extreme eastern end of
the transect, and within the western 12 km of the transect, metasedimentary units include quartzite, quartzmuscovite schist, and garnet-biotite schist. These rocks
occur in a stack of west-vergent, overturned folds (Fig.
I 1), which may be parasitic to larger overturned folds.
Intrafolial isoclinal folds with limbs parallel to gneissosity locally occur. West-vergent isoclinal folds are
locally refolded by mesoscopic, late-stage, east-vergent
folds.
A zone of ductile extension was identified near the
eastern end of the transect characterized by several
hanging-wall-down shear zones 2-3 m wide (Fig. 11 ).
These structures strike NNW and dip 60-65 ° ENE. The
hanging-wall-down sense of shear was unequivocally
indicated by the geometry of asymmetric porphyroclasts, S-C fabrics and small asymmetric folds within
the shear zones. These shear zones appear to be part of
a belt of ductile extension that continues along strike
to the northeast where we have identified extensional
shear zones of a similar character north of the town of
Sobralia (Fig. 3). Locally, extensional shear zones
occur within larger zones of west-directed contractional shear and thus may represent reactivation of earlier-formed thrusts.
5. Sobralia roadcuts
A detailed structural study made in an area of continuous highway cuts 8 km southeast of Sobralia along
2
W.D. Cunningham et al./Precambrian Research 77 (1996) 1-15
~
/
"
+
I
l
•
+ .-
?)
+
•
\1+
\
"/
+
J
= ELONGATION LINEATION ]
=MINORFOLDAXtS
J
,
/
' ~
,.- .: ~ ' ~ ' ~ -
"
"~
O.,E.TAT,ONOF00.0.0.
....
~ - - -= - ~ ; < ~ . ~ -
. , , . ~ - " ~ - ~
~
~ x - = pegmatite
~'--
mylonite zone; inferred where outside of outcrop
.
/~T~;'~
~
~
.
.
.
.
~-
.
.
.
S 0W
0L~ ,
meters
NO0 E
SO0 E
~
Fig. 12. Sketch of mylonitic shear zones and large sheath folds exposed in readout 8 km southeast of Sobralia on BR Route 116. Location of
figure shown in Fig. 3.
Route 116 (Fig. 3) provides a smaller-scale analogy to
the three-dimensional structural style tbund along the
larger transects. In these outcrops, heterogeneous biotite-garnet gneiss of probably sedimentary origin contains several mylonitic, top-to-the-west, contractional
shear zones. Between the shear zones are impressive
sheath folds with wavelengths on the order of 100 m
(Figs. 12 and 13 ). Fold axes are approximately parallel
to stretching lineations found within the mylonitic
shear zones (Fig. 12). This geometry indicates that
between the thrust horizons the metasedimentary units
were ductiley sheared, attenuated and turbulently
folded.
6. Interpretation of deformational style
Both transects record dominantly west-directed
overthrusting of rigid basement blocks over metasedi-
Fig. 13. Sheath-like eye from folds exposed in roadcut 8 km southeast of Sobralia (area of Fig. 12).
W.D. Cunningham et al. / Precambrian Research 77 (1996) 1-15
NW
NORTHERN TRANSECT
.......
.....
:
L
~
~
...... _
~
Studied ....
.......
!", ""--: "-,"
" " ' " J ' ' : [ ' ~ ' ~ ' J
" ~
~'~
~"
SE
~~××x××××××××x××××××××x×××××××~
-
"~"-j~f~
~ . . ",..,~<.......~'-,~
.. - ' ] ] = - : _
><:-~'.-.:'~
~
~
=~°~'-~
~",~~xxxxxxxxxxxxxxxxxxxxxxxxx~
~~-'~---.~-xt.~x
x x x x x x x x x x x x x x x x x x x x x ~4
[..... G............ ?-. 'W..~.~.... ,
[
13
"
SOUTHERN TRANSECT
~-Southem' Transect/
m
....,,=
~:]
"~xxxxxxx~l
"~T-,~:~:-,]~.~:;*"~:',..7',
"," ',, %<:L ¢ / X X X X ~ ' ~ " - ~ X X ~
~ . ~ , , ~ ' T ~ C h a r n o c k i t i c Gneiss ":"~ ~ ~ ~< ~( ~ ~ ~ ~--'--~..~
MetasedimentarySequences
Basement Gneisses
o
,o
Kiiometers
Fig. 14. Simplified block diagrams of northern and southern transects and possible interpretation of how transects are structurally related.
Diagram shows geometry of alternating rigid crystalline sheets and highly folded metasedimentary sequences that is characteristic of the geology
in the region.
mentary sequences (Figs. 4 and 11). Effectively, the
metasedimentary units acted as mechanically weak
glide horizons at the base of large basement thrust
sheets. In the northern transect, thrust contacts are generally shallowly dipping and are parallel to the adjacent
foliation in the basement gneiss sheets. In the southern
transect, inferred thrust contacts may be moderate to
high-angle if they are parallel to the steep foliation that
occurs within the adjacent charnockite masses. The
southern transect differs from the northern transect in
that it contains charnockite and other granulite units
that appear to be structurally repeated by moderately to
steeply dipping faults, whereas the northern transect is
characterized by wide zones of shallowly east-dipping
coherent crystalline thrust slices.
The two transects can be linked along-strike by the
apparently continuously exposed Dom Silverio metasedimentary sequence that cuts through the northern
transect and that bounds the western end of the southern
transect (Fig. 14). Regional map patterns (Radambrasil, 1983) and our structural studies carried out to
the south indicate that deeper crustal levels of highergrade units are exposed in the south and that, regionally,
structures plunge gently to the north. Thus, the northern
transect represents a shallower structural level than the
southern transect and the ground surface between the
two regions is a diagonal slice through the crust.
Fig. 14 illustrates a possible interpretation of how
the two transects are structurally related. We suggest
that the repeated charnockite blocks comprise a duplex
in which individual horses are bordered by steep faults.
This duplex may exist at depth beneath a large thrust
sheet of Piedade Gneiss and therefore the charnockite
slices are not exposed along the northern transect. The
thrust at the base of the Piedade Gneiss sheet may root
east of the southern transect. A component of sinistral
strike-slip motion during thrusting in the northern transect and evidence for dextral strike-slip motion within
the Dom Silverio metasedimentary sequence may have
affected the widths of lithologic belts within the region,
and prevents complete balancing of the cross-sections.
However, the lateral juxtaposition of isoclinally folded
and sheared metasedimentary units between thrust
slices of high-grade basement suggests enormous values of crustal shortening, certainly in excess of 50%.
The overall west-vergent thrust geometry of the region
appears to have been modified by post-contractional,
east-side-down, ductile-brittle extensional faulting,
perhaps representative of a regional-scale extensional
collapse event.
7. Discussion
Thrusting in the northern transect must be either
contemporaneous with or younger than the Neoproterozoic GalilEia batholith because the batholith is
involved in thrusting. Thus, thrusting in the northern
transect must be Brasiliano in age. If thrusts in the
northern domain are linked to those of the southern
14
W.D. Cunningham et al. / Precambrian Research 77 (1996) 1-15
domain, then the thrusts mapped in the southern domain
are also Brasiliano. This proposal is supported by the
Brasiliano R b - S r and K - A r dates that have been
obtained from units that contain penetrative fabrics
related to west-directed thrusting in the region. In this
context, it is likely that older initial ages and relict
fabrics and folds within the basement gneiss sheets are
evidence that the 2.0 Ga Transamazonian event also
affected the region. Thus, we suggest that the basement
was affected by or formed during the Transamazonian
orogeny and has been substantially to entirely overprinted by Brasiliano metamorphism and deformation.
If the metasedimentary units that form the glide horizons between the basement thrust slices are high-grade
metamorphic equivalents to the Espinhago Supergroup
metasediments and their more distal protolith equivalents, then the region was a rift basin or passive-margin
basin prior to Brasiliano tectonism (Porada, 1989).
Thus, the Brasiliano thrusting event is interpreted to
have inverted the Mid-Neoproterozoic rift basin that
formed the eastern border to the southern Sao Francisco
Craton. Although meta-ultramafie and meta-volcanosedimentary assemblages have been identified to the
north and have been interpreted as oceanic crust and
ocean floor deposits (Pedrosa-Soares et al., 1992), no
similar assemblages were identified in the area covered
in this report, and thus no obvious suture zone of Brasiliano or Transamazonian age was identified in this
study. However, talc schists that replaced serpentinitecontaining relict chrysotile veins were identified within
the Dom Silverio group metasedimentary sequence
southwest of Governador Valadares. Elsewhere, the
Dom Silverio group contains talc bodies, manganeserich sediments (gondites), mafic and rare ultramafic
units, and mixed metasedimentary lithologies (JordtEvangelista et al., 1990; Brandalise, 1991; Jordt-Evangelista, 1992). The tectonic significance of these
assemblages is not yet clear, but they may represent a
subduction complex. Thus if a cryptic suture zone of
either Transamazonian or Brasiliano age exists in the
area studied in this report, it most likely occurs within
the Dom Silverio Group that crops out to the west and
southwest of Governador Valadares.
Finally, the style of overthrusting documented in this
s t u d y - - l a r g e coherent crystalline blocks that have
overthrusted mechanically weak and intensely
delbrmed metasedimentary u n i t s - - m a y be a common
process at deep levels during rift basin closure and
inversion. M o d e m areas of active basin closure such as
the Persian Gulf and Zagros belt in the Middle East and
the northern Gulf of Carpenteria region where New
Guinea is colliding with the northern continental margin of Australia may contain similar basement/cover
sediment thrust geometries at depths of 15-20 km.
Acknowledgements
This work was funded by N S F Grant INT-9301292
to W.D. Cunningham and S. Marshak. F. A l k m i m
received support from CNPq proj. 915 153/90-6
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