DOMAINAL DISTRIBUTION OF DEFORMATION MICROSTRUCTURES IN LATE VARISCAN SC-TYPE SHEAR ZONES IN CENTRAL IBERIAN GRANITOIDS
Miguel de las Doblas Lavigne. Científico Titular del CSIC, Instituto de Geociencias (CSIC-UCM), Facultad de Medicina (Edificio Entrepabellones 7 y 8),c/ del Doctor Severo Ochoa 7, Universidad Complutense de Madrid, Ciudad Universitaria, 28040 Madrid, España, Email: doblas@mncn.csic.es
ABSTRACT
The analysis under the microscope of late Variscan SC-type deformed granitoids in Central Iberia reveals the existence of a “Domainal Distribution of Deformation Microstructures (DDDM)”. These DDDM can be classified into three different varieties: 1) Banded, characterized by a compositional and/or tectonic banding parallel to the major shearing planes; 2) Longitudinal, defined within a specific deformation band by contrasted domains bearing different compressional versus extensional microstructures; 3) Punctual, arising from the disturbant presence of large porphyroclasts in the fault rocks. These DDDM can be associated to the partitioning of deformation into compressional-/versus/extensional-related domains.
INTRODUCTION
The fault rocks analyzed here have been collected in
ductile to ductile/brittle shear zones and in brittle faults from the Spanish
Central System. These areas are mainly composed by granitoids which underwent
several extensional and transcurrent deformation events during late Variscan
time (Stephanian-Permian) and which constituted the field-work area of our
Doctoral Thesis (Universidad Complutense, Madrid, Spain), Master Thesis (Harvard
University, Massachusetts, EEUU) and “Tesis de Licenciatura” (Doblas, 1985, 1986a,
1990). Well-developed SC-type structures (SCTS) are found in a wide variety of
late Variscan ductile to brittle deformed granitic fault-rocks of our study
area in Central Iberia: mylonites, ultramylonites, cataclasites,
ultracataclasites, fault gouges, slickenside surfaces, etc.
Since the SC nomenclature was introduced by Berthe et al (1979),
much has been written about these structures, particularly regarding their
origin, types, kinematics, etc.: Platt & Vissers, 1980; White et al., 1980;
Simpson & Schmid, 1983; Lister & Snoke 1984; Shimamoto, 1989; Rykkelid
& Fossen, 1992). Several of our early scientific contributions related to our
Master Thesis at Harvard University were the first to introduce the SC
terminology in Spain (Doblas, 1985, 1986b, 1987, 1988; Doblas et al., 1983,
1988; Fernández & Doblas, 1987). SCTS are defined in the present manuscript
as composite planar fabrics that are reliable kinematic indicators, constituted
by an “S-type plane” (STP) and a “C-type plane” (CTP) and whose terminology is
highly variable (Berthé et al., 1979; Logan et al., 1979; Platt & Vissers,
1980; Rutter et al., 1986; Dennis & Secor, 1987; Doblas, 1987, 1990): 1)
Under ductile conditions, an STP might involve an S plane or a mylonitic
foliation, while its brittle equivalent is a P plane; 2) A CTP under ductile
conditions is represented by a wide variety of planar structures such as C
planes, extensional crenulation cleavages 1 (ecc1, shear bands, normal slip
crenulations NSC or C’ planes), extensional crenulation cleavages 2 (ecc2) and
reverse slip crenulations (RSC or inverse microfaults), while its brittle equivalents
are Y planes and Riedel R/R’ fractures.
Partitioning of deformation and fabric heterogeneity are
essential characteristics of shear zones (Lister & Williams, 1983). The
concept of domainal differentiation of deforming rocks is now widespread,
including extension-/contraction-related domains (Platt, 1984; Dennis &
Secor, 1987; Marcoux et al., 1987), crystallographic fabrics (Fueten et al.,
1991) and textural characteristics of some fault rocks (Bobyarchick, 1991).
According to this last author, in “domainal mylonites”, deformation is
partitioned into discrete shear domains such as shear bands.
The present paper intends to develop such concepts as the geometrical arrangements of SC-type structures, the partitioning of deformation and the domainal distribution of certain microstructures, applying them to a wide variety of late Variscan fault rocks of Central Iberia analyzed under the microscope.
DOMAINAL DISTRIBUTION OF DEFORMATION MICROSTRUCTURES
The concept of “Domainal Distribution of Deformation
Microstructures” (DDDM) refers to the differentiation of the fault rocks into
microscale domains bearing contrasted deformation features such as SC-type
planar structures (SCTS), deformed porphyroclasts, asymmetric microfolds,
microduplexes and what we call “trains of grains”.
The Central Iberian
SC-sheared granitoids display different types of DDDM, which can be
geometrically classified into three major groups: 1) Banded (parallel to the
shearing planes); 2) Longitudinal (within a specific deformation band); and, 3)
Punctual (in the vicinity of large porphyroclasts which constitute disturbing
rigid objects).
Thin-sections were cut
parallel to the stretching lineations and perpendicular to the shearing planes.
The exceptionally large microphotographs used to design our figures were
captured with a special petrographic microscope incorporating an automatic printer
machine that allowed us to obtain excellent black & white images of the
whole field of view. We were able to use it in the personal office of Professor
John Ramsay, our supervisor during our 1987/1988 semester-training-stage at the
Eidgenössische Technische Hochschule (ETH, Zürich) with a Grant of the European
Union.
Figure 1. Tracings of photomicrographs showing three
examples of Banded DDDM in
granitic mylonites (a & b) and an ultramylonite (c) with different types of
SCTS. The different bands or domains are highlighted outside the drawings with
numbers (1, 2 and 3 in a and c) or letters (S and C in b). C: C plane. C’: C’
plane. F: Feldspar. I: Inverse microfault. M: Asymmetric microfold. MF:
Mylonitic foliation. Q: Quartz. S: S plane. Semiarrows indicate the sense of
movement. Scale bars: 1 mm. See text for more details.
1) Banded DDDM (Fig. 1):
This type of DDDM is defined by the existence of a
compositional/tectonic banding parallel to the movement planes, triggering
alternating domains with contrasted deformation structures (Fig. 1).
Composition plays a major role in the establishment of
banded DDDM in the granitic fault rocks of the studied area. This is
exemplified by the SC-deformed granite of figure 1a which shows a domain 1
constituted by quartz-feldsparic-rich zones (with dominant contraction-related
S flattening planes accompanied by poorly-spaced C planes), contrasting with a
mica-rich layer (with a closely-spaced
mylonitic foliation transected by extensional C’ planes; domain 2). These two
domains, which are usually believed to indicate low and high strains, clearly
show partition of contraction-/extension-related structures.
Classical SC sheared granitoids constitute excellent
examples of banded DDDM (Fig. 1b), with “S domains” characterized by sigmoidal
S planes bounding/shaping feldspar porphyroclasts and quartz ribbons and “C
domains” with closely-spaced shearing planes in phenocryst-free mica-rich
layers. Here again, composition and deformation are important factors in the
generation of this type of DDDM.
Spectacular examples of banded DDDM resulting from the
partition into different domains of contraction-/extension-related structures
are usually found in ultramylonites. In this sense, figure 1c shows a domain 1
with a mylonitic foliation transected by abundant C’ extensional planes,
contrasting with a domain 3 characterized by contractional structures such as
asymmetric microfolds (usually arranged in fans bounded by inverse
microfaults). Exceptionally, domain 1 displays an isolated fan of asymmetric
microfolds, whose generation might be explained in relation to the
porphyroclast on its right (as we will see later, this is a punctual DDDM).
Intermediate domain 2 can be related to compositional factors as it corresponds
to a mica-rich band where a more closely-spaced mylonitic foliation is
transected by abundant extensional C’ planes.
2) Longitudinal DDDM (Fig. 2):
Longitudinal DDDM are defined within a specific deformed
zone (parallel to the movement planes), by lateral transitions involving
contrasted domains with contraction-/extension-related deformation structures
(Fig. 2): respectively, inverse microfaults, asymmetric microfolds,
microduplexes or P planes; and, C’ planes or R Riedel fractures. Spectacular
examples of longitudinal DDDM might be found under highly different deformation
conditions, as examplified by ductile ultramylonites (Fig. 2a) or brittle
slickenside surfaces (Fig. 2b).
We suggest that these contrasted domains bearing
contraction-/extension-related structures might indicate, respectively,
thickening and thinning sectors within a deformation zone. According to Dennis
& Secor (1987), normal and reverse slip crenulations (NSC and RSC;
respectively equivalent to C’/R extensional planes and P planes or I inverse
microfaults, in figure 2), act to maintain the initial thickness of the
deformation zone, compensating thickening (by means of NSC) and thinning (by
means of RSC). However, the slickenside surface of figure 2b contradicts these
ideas, showing that: 1) thickness is not maintained; and, 2) thinning and thickening
of the deformation zone are respectively associated to extensional R fractures
(equivalent to NSC) and to contractional P planes (equivalent to RSC). Note
that the thickening observed in this example is enhanced by the effect of two
generations of P planes (P1 and P2).
The alternation and coexistence of such
contraction-/extension-related domains along specific deformation zones has
been described from the microscale (Liu, 1990), to the outcrop scale (Marcoux
et al., 1987) and even at the macroscale of an orogenic belt such as the Alps
(Ratschbacher et al., 1989).
3) Punctual DDDM (Figs. 3 & 4):
This type of DDDM is found in the vicinity of
porphyroclasts which constitute disturbing/discrete rigid objects within the
mylonitic rock. Porphyroclasts have a major influence in the establishment of
domainal heterogeneities in deforming rocks: crenulations (Hanmer, 1979), drag
patterns (Ghosh & Ramberg, 1976), asymmetric folds (Bjornerud, 1989) or
σ-/δ-type porphyroclasts (Passchier & Simpson, 1986; Van Den Driessche
& Brun, 1987). In particular, their distribution, abundance, size and
irrotational/rotational characteristics, determine the type of SC structures
that might develop.
The effect of porphyroclasts in the closest structural
elements is best illustrated in granitic ultramylonites with a well-developed
mylonitic foliation and abundant porphyroclasts (Fig. 3). In this case,
equidimensional rotating/sliding porphyroclasts might be observed disturbing
the predominant foliation in their frontal parts, by means of fans of
asymmetric microfolds, sometimes bounded by inverse microfaults (1, 2 and 3 in
figure 3). Neither of these porphyroclasts can be clearly assigned to the
σ-/δ-types (Passchier & Simpson, 1987) and they certainly do not possess
the geometry of typical rolling structures (Van Den Driessche & Brun,
1987). However, they might represent porphyroclasts with a more complex history
(σ followed by δ or the opposite) or having reached a stable position at high
strains (Passchier, 1987). A spectacular case is constituted by porphyroclast 2
whose continued movement/deformation triggers two contrasted punctual DDDM in
its frontal region: a set of C’ extensional planes in its lower part and a major
fan of asymmetric microfolds in its upper sector. This might be understood in
terms of a slowly rotating/sliding porphyroclast generating a frontal
“sledge-effect”, partitioning deformation into contractional (a wedge-shaped
fan of asymmetric microfolds bounded by inverse microfaults) and extensional (a
set of C’ planes) structures. These fans of microfolds are kinematically
similar to the asymmetric folding or drag patterns of layering around rigid
objects depicted by Ghosh & Ramberg (1976) and Bjornerud (1989), or to the
reverse slip crenulation (RSC) of Dennis & Secor (1987).
Porphyroclasts often breakup into several fragments, thus
giving rise to a particular type of punctual DDDM which might be called
“trains-of-grains”, corresponding to the disaggregation of these rigid objects
into stretched trains of pull-aparts, shear-steps or σ-/δ-type fragments (Fig.
4). Additionally, the different modes in
which porphyroclasts breakup into “trains-of-grains”, are controlled either by
the S or the C planes (depending on their size, location or orientation; Fig.
4). Ghosh & Ramberg (1976) used a similar terminology (“trains of rigid
inclusions”) to refer to experimentally produced models.
Figure 4. Tracings of photomicrographs showing four examples of Punctual DDDM in granitic
mylonites (a, c & d) and ultramylonites (b) with different types of SCTS
arising from the disaggregation of porphyroclasts into “Trains of Grains” (TG).
The TG highlighted correspond to fragments of σ phenocrysts (1, 2, 9, 10, 11),
δ phenocrysts (3), moderate to extreme pull-aparts (4, 5, 6) and domino-type
shear-steps (7, 8). Additionally, the TG fragments might be controlled by C
planes (1, 10, 11), S planes (6, 7, 8, 9) or mylonitic foliations (3, 4, 5). C:
C plane. C’: C’ plane. ECC2: extensional crenulation cleavage. M: Asymmetric
microfold. MF: Mylonitic foliation. S: S plane. SS: shear-step. σ & δ: σ-
& δ-type phenocrysts. Semiarrows indicate the sense of movement. Scale
bars: 1 mm. See text for more detail.
CONCLUSIONS
The “Domainal Distribution of Deformation
Microstructures” (DDDM) is a well-established characteristic of the Central
Iberian SC sheared granitic fault rocks and mylonites.
Three different types of DDDM have been described: 1) Banded, defined by the existence of
a compositional/tectonic banding parallel to the shearing planes, resulting in
a series of alternating layers characterized by contrasted deformation
structures (SCTS, asymmetric microfolds or inverse microfaults); 2) Longitudinal, defined within a
specific deformation band by lateral transitions from contraction- to
extension-related domains, which might be interpreted, respectively, in terms
of thickening and thinning of the deformation zone; and, 3) Punctual, which can be related to
the presence of porphyroclasts acting as discrete/disturbant rigid objects.
Different types of DDDM might be found together in a same
sample, as in the ultramylonite of figure 1c which displays banded and punctual
structures. A three-dimensional analysis of these fault-rocks would probably
reveal many new DDDM to add to the ones described here.
The existence of the DDDM seems to be independent of the
degree of deformation (similar domains with SCTS are found in ultramylonites,
mylonites, etc.), the regime (ductile or brittle) and the scale.
These domainal distributions might be explained in terms
of deformation partitioning, leading to the establishment of contrasted domains
bearing contraction-/extension-related structures. Additionally, the different
domains triggered by the DDDM bear complementary shear sense indicators, which
might be useful to unravel the kinematics of a given deformed rock.
Finally, some of the microstructures found in these
Central Iberian fault rocks have rarely been described in the literature: fans
of asymmetric microfolds, “trains-of-grains”, microduplexes, etc.
Acknowledgements: We are indebted to John Ramsay for his valuable supervision during our stage at the ETH. We acknowledge useful comments by Carol Simpson, Stefan Schmid, Neil Mancktelow, Dorothee Dietrich and José Luis Hernández Enrile. We thank José Arroyo for drawing the figures.
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