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Journal of South American Earth Sciences 16 (2003) 193–204
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Mazatan metamorphic core complex (Sonora, Mexico): structures along
the detachment fault and its exhumation evolution
Ricardo Vega Granilloa,*, Thierry Calmusb
b
a
Departamento de Geologı́a, Universidad de Sonora, Hermosillo, Sonora, Mexico
Instituto de Geologı́a, Estación Regional del Noroeste, UNAM, Hermosillo, Sonora, Mexico
Received 31 March 2003; accepted 30 April 2003
Abstract
The Mazatán Sierra is the southernmost metamorphic core complex (MCC) of the Tertiary extensional belt of the western Cordillera. Its
structural and lithological features are similar to those found in other MCC in Sonora and Arizona. The lower plate is composed of
Proterozoic igneous and metamorphic rocks intruded by Tertiary plutons, both of which are overprinted by mylonitic foliation and N708Etrending stretching lineation. Ductile and brittle– ductile deformations were produced by Tertiary extension along a normal shear zone or
detachment fault. Shear sense is consistent across the Sierra and indicates a top to the WSW motion. The lithology and fabric reflect
variations in temperature and pressure conditions during extensional deformation. The upper plate consists mainly of Cambrian –
Mississippian limestone and minor quartzite, covered by upper Cretaceous volcanic rocks, and then by Tertiary syntectonic sedimentary
deposits with interbedded volcanic flows. Doming caused uplift and denudation of the detachment, as well as successive low-angle and highangle normal faulting across the western slope of Mazatán Sierra. An 18 ^ 3 Ma apatite fission-track age was obtained for a sample of
Proterozoic monzogranite from the lower plate. The mean fission-track length indicates rapid cooling and consequent rapid uplift of this
sample during the last stage of crustal extension.
q 2003 Elsevier Ltd. All rights reserved.
Keywords: Detachment fault; Extension; Mazatán; Metamorphic core complex; Sonora
1. Introduction
During the Cretaceous and Tertiary, the tectonic
evolution of northwestern Mexico was controlled by the
interaction between the Farallon and Pacific oceanic plates
and the western margin of the North American craton. A
large increase in the convergence rate between the Farallon
and North America plates at 75 Ma (Engebretson et al.,
1985) may have caused the Late Cretaceous – Early
Tertiary Laramide orogeny, which was characterized by
intense magmatism and compressional tectonics that
involved cortical thickening (Coney and Harms, 1984;
Dickinson, 1989). During the Tertiary, the western margin
of the North American plate was the locus of complex
subduction, mainly due to the fragmentation of the Kula
and Farallon oceanic plates. Through Eocene –Oligocene
* Corresponding author. Tel.: þ 52-662-2592110; fax: þ 52-6622592111.
E-mail addresses: [email protected] (R.V. Granillo),
[email protected] (T. Calmus).
0895-9811/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0895-9811(03)00066-X
time, continuous subduction generated voluminous inland
magmatism that formed the Sierra Madre Occidental
ignimbritic belt (Clark et al., 1982). By the Late
Oligocene – Early Miocene, the volcanic front migrated to
the west. In Sonora, between 26 and 12 Ma, magmatism
was contemporaneous with late orogenic collapse throughout northwestern Mexico (Gans, 1987; Nourse et al., 1994),
which produced metamorphic core complexes (MCC) and
the Basin and Range morphotectonic province.
Tertiary extension in Sonora occurred on previously
deformed continental crust, including four tectonostratigraphic terranes (Fig. 1): (1) To the north and northeast, the
Pinal schist and younger rocks of the southwestern margin
of the North American craton consist of Proterozoic
metamorphic rocks intruded by 1.68 Ga and mid-Proterozoic plutons, overlain by upper Proterozoic and Paleozoic
strata. (2) To the northwest, in the Altar desert region, the
Papago terrane (Haxel et al., 1980) is characterized by a
thick sequence of Early –Late Jurassic low-grade metamorphic volcanic rocks, which represents supracrustal rocks
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Fig. 1. Geologic map of the Mazatán Sierra. (1) Lower Proterozoic metamorphic rocks; (2) middle Proterozoic monzogranite; (3) Paleozoic limestone; (4)
Upper Cretaceous-Paleocene volcanic and volcanoclastic rocks; (5) Paleocene calc-alkaline grandioritic intrusion; (6) Tertiary two-mica granite; (7) Lower
Miocene (?) detrital Belleza Formation; (8) Mio-Pliocene detrital unit; (9) rhyolitic flow; (10) basaltic-andesite flow; (11) alluvial fan; (12) low-angle normal
fault; (13) high-angle normal fault; (14) strike and dip mylonitic foliation; (15) zone with ductile strain. For A–A0 geologic section, see Fig. 7; B –B0 geologic
section, see Fig. 9. Insert: (16) main MCC from Arizona and Sonora. NAC, North American craton; SPT, Papago terrane; CT, Caborca terrane; CoT, Cortés
terrane; H, Hermosillo; MK, Magdalena de Kino; Cab, Caborca; and G, Guaymas.
of the magmatic arc accreted on the southwestern margin of
the craton (Busby-Spera, 1988). Proterozoic and Paleozoic
rocks are scarce and locally thrusted on Jurassic units
(Calmus and Sosson, 1995; Caudillo-Sosa et al., 1996),
similar to the pattern of structures described in the
Quitobaquito Hills area of southwestern Arizona (Haxel
et al., 1984). (c) South of the Papago terrane, the Caborca
terrane consists of a high-grade metamorphic lower
Proterozoic basement intruded by the 1100 Ma Aibo granite
(Anderson and Silver, 1981) and unconformably overlain by
more than 3000 m of upper Proterozoic – Cambrian shallowwater sequences (Stewart et al., 1984). Triassic – Upper
Jurassic marine fossiliferous deposits unconformably overly
older rocks. (d) South of the Caborca terrane, the Cortés
terrane is made up of lower Paleozoic deep marine
sequences deposited along the margin of the North
American craton; these rocks were thrust on coeval shelf
deposits of the craton at the end of the Paleozoic. Although
Basin and Range tectonics extend to the south into the
Trans-Mexican volcanic belt and probably into Oaxaca
(Henry and Aranda-Gómez, 1992, 2000), MCC are not
known south of the limit between the Caborca and Cortés
terranes, which suggests that the Tertiary crustal extension
is partly controlled by inherited structures and an overthickened crust (Coney and Harms, 1984), as in the North
American Cordillera, where MCC are located only west of
the Overthrust belt.
The first detailed studies of MCC were performed in the
Basin and Range province (Coney, 1980), which appear in
Arizona and southeastern California (Whipple, Buckskin,
R.V. Granillo, T. Calmus / Journal of South American Earth Sciences 16 (2003) 193–204
Harquahala, and Catalina Mountains; Davis et al., 1980) and
along a belt in the North America Cordillera from southwestern Canada to Sonora. Previous structural studies,
radiometric ages, and thermochronologic studies of MCC
provide precise constraints on their thermotectonic evolution (Rehrig and Reynolds, 1980; Rehrig et al., 1980;
Wernicke, 1992; Foster et al., 1993; Fayon et al., 1996). In
Sonora, Coney (1980) identifies four MCC: Magdalena,
Madera, Mazatán, and Pozo Verde Sierras. The Aconchi
Sierra, located 75 km north of Mazatán Sierra, also exhibits
in its northeastern area structural characteristics of MCC
(Nourse et al., 1994; Calmus et al., 1996; Rodrı́guez-Castañeda, 1999). Structural and petrologic studies have
focused mainly on the Magdalena MCC (Nourse, 1990)
and the Mazatán Sierra MCC (Richard, 1991; Vega-Granillo, 1996a,b). In Arizona, the stretching rate of the midTertiary pre-Basin and Range extension might locally
exceed 100% (Hamilton and Myers, 1966; Davis, 1983).
The presence of probable Proterozoic crystalline basement
in the footwall of Mazatán Sierra, as in the southeast
Arizona Santa Catalina MCC (Precambrian Oracle granite
and Pinal schist), may suggest a similar important stretching. Continental deposits related to syntectonic basins
located on the hanging wall of MCC are terrigenous and
lacustrine with interlayered volcanic rocks, including sparse
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nonmetallic deposits of gypsum, borates, and zeolites
(Miranda-Gasca et al., 1998).
2. Pre-Oligocene geology of the Mazatán Sierra
The Mazatán Sierra is located in central Sonora (Fig. 1),
almost 70 km east of Hermosillo. The range is roughly
circular, with a diameter of 15 km and an elevation of
1000 m above the surrounding plains. It is classically
considered the southern end of the Upper CretaceousPaleocene Aconchi batholith, though it is isolated from the
main range. The oldest rocks are gneisses, amphibolites,
mica schist, and quartzite that form blocks that are included
as xenoliths in a monzogranite (Fig. 2a). One xenolith
located at an elevation of 1180 m in the Bachán creek
displays an unconformity between migmatitic gneisses and
metasedimentary rocks. The metamorphic rocks can be
correlated with the Lower Proterozoic (, 1700 Ma) basement of the Caborca terrane by their lithological character
and amphibolite facies metamorphism; metasedimentary
rocks can be correlated with Neoproterozoic rocks widespread in northwestern and central Sonora.
The metamorphic rocks are intruded by a porphyritic
monzogranite of uncertain age. An Rb/Sr age of
Fig. 2. (a) Migmatitic quartz-feldspathic gneiss with ptygmatic microfolding as xenolith in porphyritic mylonite monzogranite. An anatexitic layer was formed
around the contact. (b) Ultramylonite horizon within porphyritic mylonite monzogranite that wedges laterally and has sharp contacts. (c) Oblique folds in the
ultramylonitic zone. Dark ultramylonite bands alternate with mylonite monzogranite bands. Li indicates stretching lineation; view to the NE. (d) Highly strained
zone with folded boudins (1 and 2) of mylonite monzogranite between ultramylonite bands (3). Li indicates stretching lineation direction; view to the N. All
exposures crop out along the Cañada Bachan on the western slope of Mazatán Sierra. Scales are 13 cm pen in (a) and (c) and 30 cm hammer in (b) and (d).
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1475 ^ 29 Ma (Damon, pers. comm.), which appears as
1480 ^ 30 Ma in an unpublished report, corresponds to a
granite collected near the Bachán ranch at the top of the
Sierra. A granite body of 1410 Ma was reported (Richard,
1991; after Damon, pers. comm.) northwest of the Sierra in
the foothills close to the La Feliciana ranch. These
unpublished and poorly located radiometric ages do not
conclusively establish the age of the bulk of the Mazatán
Sierra. Nevertheless, the presence of xenoliths of Precambrian gneisses and metasedimentary rocks and the lack of
Paleozoic sedimentary rocks as roof pendants in the
Mazatán Sierra, which are abundant in the Paleocene
Aconchi batholith, suggest a Proterozoic age for the
porphyritic monzogranite.
In central Sonora, the crystalline basement is covered by
Paleozoic miogeoclinal sequences that crop out as isolated
hills (Stewart et al., 1990; Richard, 1991). They contain
strata of Middle Cambrian– Middle Permian age (VegaGranillo, 1996a,b). Paleozoic rocks are unconformably
covered by an intermediate to felsic volcanic unit, which
could be correlated with the Tarahumara Formation (Wilson
and Rocha, 1946) of Late Cretaceous – Paleocene age
(McDowell et al., 1995, 2001). In central Sonora, the
Tarahumara Formation is mostly older than the calcalkaline dioritic – granitic intrusive rocks of the Sonoran
batholith (McDowell et al., 2001). A porphyritic granodiorite from northwest of the Mazatán Sierra, close to El
Garambullal ranch, yields a U – Pb age of 58 ^ 3 Ma
(Anderson et al., 1980). A mid-Tertiary two-mica peraluminous granite was emplaced over Middle Proterozoic –
Lower Paleocene crystalline rocks and under Paleozoic
limestone. A leucocratic dike, presumably associated with
this intrusion, yields 33.0 ^ 8 Ma (Damon, pers. comm.).
3. Structures in Mazatán Sierra MCC
Three structural features can be recognized in the
Mazatán Sierra MCC: (1) shear zone with dynamic
metamorphism, (2) local doming, and (3) brittle normal
faulting.
The metamorphic core of the Mazatán Sierra, similar to
that of many other western Cordillera MCC, is characterized by igneous and regional metamorphic rocks overprinted by dynamic metamorphism related to a ductile to
brittle– ductile shear zone. The core corresponds to the
lower plate (footwall) of the MCC (Coney, 1980) and the
shear zone to a detachment fault. Due to the movement
along the shear zone, mylonitic foliation (S2) and
stretching lineation were developed in Proterozoic
crystalline rocks, as well as in upper Cretaceous and
Tertiary intrusive rocks. The most common rocks in the
shear zone are mylonitic to protomylonitic granite derived
from Proterozoic two-mica porphyritic granite. The
typical fabric in this protomylonitic granite is shear
band cleavages (S-C0 and S-C) with schistosity formed by
dynamically recrystallized quartz lenses and preferred
orientation of biotite, diagonally transected by discontinuous extensional shear bands (C0 ).
Structures and microstructures of the dynamically
metamorphosed rocks give evidence for a shear zone
along which deformation and differential displacement
were produced by ductile flow. The main deformation
mechanism in mylonitic gneiss was quartz deformation by
intracrystalline slip with recovery and dynamic recrystallization. Quartz deformation allows the rotation of planar or
prismatic minerals, which enhances foliation development.
Feldspars are mainly deformed by cataclasis and intracrystalline slip and display dynamic recrystallization. Muscovite
and local epidote are neoformed. Dynamic recrystallization
in quartz occurs at temperatures higher than 300 8C,
whereas in feldspars, this mechanism is important above
400– 500 8C (Passchier and Trouw, 1998). Deformation
temperatures and the few neoformed metamorphic minerals
suggest greenschist facies conditions during the dynamic
metamorphism event.
Within the shear zone, deformation is relatively homogeneous, with the exception of a few meters of thick discrete
zones, where mylonitic granite is gradually transformed into
stripped gneisses with isoclinal microfolding. Dynamically
metamorphosed rocks form the bulk of Mazatán Sierra;
however, highly strained rocks are best exposed along the
western slope and foothills. On the basis of the relationships
between the topography and outcrops, we estimate that the
shear zone thickness exceeds 200 m.
Thin bands of mylonite and ultramylonite occur within
the mylonitic granite, typical of highly strained zones
described in other MCC. The rocks are black, coherent, and
very fine grained (chert-like), sometimes with feldspar
bands and porphyroclasts, as well as neoformed muscovite
in foliation planes. The matrix is formed by fine
(, 0.1 mm), dynamically recrystallized grains of quartz,
feldspar, and mica. Ultramylonite bands have straight
contacts with the mylonitic granite host and wedge out
laterally (Fig. 2b).
The regional metamorphic foliation (S1) in Proterozoic
rocks tends to be parallel to the shear plane. Consequently,
a shear cleavage develops in those rocks. However, in
some places, a mylonitic foliation overprints S1 and F1
folds. The F2 folds, which are generated by the shear
movement, are well represented in amphibolite and schist
that display two phases of isoclinal to tight decimetric
folds, the latter with recumbent folds whose axial planes
are subparallel to the shear plane. In some places,
ptygmatic folding is associated with shear deformation of
the heterogeneous mix of xenoliths and host rocks (Fig. 2a).
Proterozoic xenoliths, some of tens meters along, outcrop
like boudins in the mylonitic granite host. Quartzite,
amphibolite, schist, and gneiss show dynamic recrystallization in the shear zone, but in areas outside the dome, the
quartzite has a laminated fabric with polygonal grains.
In the bulk of Mazatán Sierra, stretching lineation defines
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Fig. 3. Equal area lower hemisphere projections of stretching lineation in
(a) mylonitic granite and (b) mylonite and ultramylonite bands. (c) Equal
area lower hemisphere projections of fold axes in mylonitic granite,
mylonite, and ultramylonite. (d) Lower hemisphere projections of
mylonitic foliation poles in mylonitic rocks, measured within the entire
dome. n ¼ 30 in (a) –(c) and 55 in (d).
a shear direction between 60 and 908NE with an average of
70 and 808NE (Fig. 3a and b). This shear direction
coincides with those defined in other MCC in Sonora
(Nourse, 1990; Nourse et al., 1994) and Arizona (Keith
197
et al., 1980) and thus suggests a coherent extension trend
through the south Basin and Range provinces. Mylonitic
granites have parallel quartz-feldspar bands, centimeters to
decimeters thick, which could be formed by isoclinal to
subisoclinal folding of leucocratic dikes. Mylonitic to
ultramylonitic bands, some thicker than 10 m, are highly
strained in the shear zone. In these zones, centimeter-scale
ultramylonite bands are intercalated with mylonitic
gneisses and granites (Fig. 2). Common structures inside
them are sheath, oblique, and rootless folds; boudinage;
folded boudins; and superposed folds. Tubular morphology
of sheath folds was observed in a few exposures (Fig. 4).
Decimetric oblique folds vary from isoclinal to open.
Isoclinal and subisoclinal fold axes in mylonitic and
ultramylonitic rocks trend between N60 and 908E and
N70 and 908W (Fig. 3c), partially coincident with the
stretching lineation trend or the maximum elongation
direction, typical in sheath or oblique folds. A pattern of
fold interference exists in some ultramylonitic rocks and
corresponds to a combination of a domed, crescent
mushroom pattern and a convergent –divergent pattern
(Ramsay, 1967; Ramsay and Huber, 1987). The coexistence of both fold types in the same rock can be attributed
to the bending of sheath folds.
Ultramylonitic zones in the lower part of the Bachán
creek enclose some folded boudins, which indicates a
progressive rotation of the strain ellipse during simple
shear deformation that induces folding of the mylonitic
bands at the same time. Rotation is required to produce
the observed change in the deformation process from
extension (boudinage) to shortening (folding) domains.
Fig. 4. Sketch of a sheath fold: (a) black ultramylonite with quartz-feldspar and quartz bands. L is the stretching lineation; (b) lateral view; (c) –(d) fold
reconstruction before erosion. X indicates the maximum elongation direction.
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Domains of extension are followed by shortening, only
in the case of polyphase deformation (Passchier and
Trouw, 1998).
Shear sense is defined by microstructures created by
noncoaxial deformation, such as oblique foliations, shearband cleavages (S-C0 , S-C), mantled porphyroclasts, mica
fish, and microfolding (Fig. 5). Shear sense is consistent
in the analyzed sites around the dome and indicates a top
to the SW sense of shear. In thin, highly strained horizons,
inversions in shear sense were detected. We theorize that
shear sense inversions could be produced during sheath fold
formation (Fig. 6).
The most accepted model to explain Cordilleran MCC
proposes that shear zone deformation corresponds to a lowangle normal fault (Wernicke, 1981, 1985; Spencer and
Reynolds, 1989) that extends through the crust or even
Fig. 5. Kinematic indicators used to define the shear sense in Mazatán Sierra mylonites: (a) oblique foliation (fs-C) in quartzite mylonite; stair-stepping of C
bands can be observed. (b) Shear band cleavage (S-C0 ) in porphyroclastic mylonite; intracrystalline sliding occurs in potassic feldspar porphyroclasts with
antithetic (at) and synthetic (st) slides. (c) Muscovite mica fish and oblique foliation (fs-C) in quarzite mylonite. (d) Delta-type feldspar porphyroclast in
laminate ultramylonite. (e) Sigma-type opaque minerals and biotite in laminated mylonite. Dark bands are dynamic recrystallized biotite; white bands are
dynamic recrystallized quartz. (f) Ultramylonite with microfolding of recrystallized feldspars bands; folding is interpreted as due to the rotation of
porphyroclasts. Rounded feldspar porphyroclasts are in a very fine recrystallized matrix. Shear sense is indicated by arrows. All thin sections are parallel to
stretching lineation and normal to mylonitic foliation. Plane light except photos (e) and (f). Scale as in (e).
R.V. Granillo, T. Calmus / Journal of South American Earth Sciences 16 (2003) 193–204
199
Fig. 6. Schematic drawing showing possible inversion in shear sense caused by oblique or sheath folding. (a) Mylonitic band with dextral shear sense
indicators; (b) oblique fold (with axis parallel to the maximum elongation), shear sense in the lower limb is sinistral and dextral in the upper limb; (c) by
comparison, isoclinal fold with N –S axis (perpendicular to the previous shear direction) does not change shear sense.
the lithosphere. Along those major faults, a spatial variation
of deformation mechanisms exists, from cataclasis in the
upper crust to intracrystalline deformation with recovery
and dynamic recrystallization in the deepest zones. A
temporal variation, other than the spatial variation previously discussed, exists in the deformational style. In the
Mazatán Sierra shear zone, the deepest zone underwent
ductile deformation with a low strain rate in the first stages
to produce mylonitic granites from granular rocks. This
strain affects a great volume of rock, because at higher
temperatures, strain is more disperse (Williams et al., 1991).
When the lower plate is uplifted to shallower depths with
lower temperatures and confining pressures, strain is
concentrated in narrow zones with high strain, which cut
previously formed mylonitic granites and produced zones of
mylonite and ultramylonite. This strain concentration
phenomenon (tectonic softening) has been noted by
Williams et al. (1991). In Mazatán Sierra, mylonitic
horizons thicker than ten meters mainly occur along
lithological discontinuities, due to differences in the
rheological behavior of rocks. When the ductile shear
zone cooled due to tectonic exhumation, brittle deformation
overprinted mylonitic zones, which was expressed as
brecciated zones with thicknesses from centimeters to tens
of meters.
In the Mazatán Sierra, no radiometric ages have been
obtained for minerals formed during dynamic metamorphism (e.g. muscovite). However, K – Ar 33 Ma pegmatitic
leucocratic dikes are strained, whereas andesitic dikes
with 21.1 ^ 5 Ma K –Ar age (Damon, pers. comm.) are
undeformed and cross through the mylonitic foliation.
According to these data, dynamic metamorphism associated with the MCC formation must have occurred during
the Oligocene –Early Miocene; this time coincides with
U – Pb, Rb –Sr, and K – Ar radiometric ages established in
MCC of Sonora and Arizona, where detailed studies
show time deformation to be Oligocene – Early Miocene
(Dickinson, 1991; Nourse et al., 1994).
4. Syntectonic clastic deposition
Tectonic unroofing of the MCC core through displacement of the upper plate along a low-angle normal fault
created a syntectonic basin, which was filled with clastic
deposits produced by the erosion of uplifted blocks. A thick
clastic series, informally named the Belleza Formation
(Richard, 1991; Vega-Granillo, 1996a), crops out west of
Mazatán Sierra along more than 12 km perpendicular to
strike, though in great part, these strata are concealed by
younger sediments. Bedding dips vary from vertical far
from the main fault to 308NE closest to the fault, which
suggests a syntectonic tilting. These strata are overlain by
alluvial deposits. We distinguish three units in the Belleza
Formation. The lowest is composed of intercalations of
conglomerate, siltstone, and sandstone with thin volcanic
flows and tuffs. Clasts in conglomerate vary from mainly
volcanic in the lower beds to limestone and granitic rocks in
the upper part. The middle unit is formed of intercalations of
sandstone and siltstone with scarce thin limestone and
gypsum beds. The upper unit consists primarily of
sedimentary breccias with andesite clasts. The Belleza
Formation rocks have sedimentological characteristics of
alluvial fans and lakes, with both fluvial and lacustrine
facies and minor volcanic and volcanoclastic contributions.
The formation was displaced over a Tertiary (?) garnetbearing two-mica granite along a low-angle normal fault
subsidiary to the detachment fault of the Mazatán MCC.
Both granite and sedimentary rocks are very brecciated
along the fault, but there is no evidence of ductile
penetrative strain in the granite. In some places, siltstones
and sandstones are deformed by subisoclinal to isoclinal
drag folds generally less than 1 m in scale. Large open folds
with large interlimb angles and NE-trending axes have been
mapped in the Belleza Formation between the Mazatán and
Puerta del Sol Sierras (Fig. 8). The intercalated andesite is
very similar to andesitic dikes crossing the lower plate; one
dike has a 22 Myr K – Ar date (Damon, pers. comm.).
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Fig. 7. Mazatán Sierra cross-section A– A0 from Fig. 1. (1) Belleza formation; (2) Tertiary two-mica granite; (3) leucocratic dykes; (4) Upper CretaceousPaleocene volcanic and volcanoclastic rocks; (5) Paleocene calc-alkaline granodiorite; (6) Paleozoic limestone; (7) Middle Proterozoic monzogranite; (8)
Lower proterozoic regional metamorphic rocks; (9) low-angle normal fault; (10) high-angle normal fault; (11) mylonitic foliation.
The Belleza Formation can be correlated with continental
sequences with minor volcanic contributions, which have
been associated with exhumation of MCC. These regions in
Sonora include the Tubutama, Magdalena, and Aconchi
areas (Frye, 1975; Roldán-Quintana, 1979; Paz, 1988;
Nourse, 1989; Miranda-Gasca and De Jong, 1992).
Palynological studies in Tubutama and Magdalena indicate
Oligocene – Miocene and Miocene – Pliocene ages, respectively (Richard, 1991). Volcanic rocks beneath the clastic
sequence in Magdalena yield a K – Ar age of 27 Ma;
intercalated basaltic andesite indicates a K –Ar age of
22 Ma (Miranda-Gasca and Gómez-Caballero, 1993).
The lithological character of the Belleza Formation, the
basal low angle normal fault, the presence of syngenetic
conjugate faults, slumps folds, drag folds and the
systematic dip variation toward the main fault, indicate
that sedimentation and extensional deformation were
contemporaneous, at least during the later stages of
MCC formation.
was deposited. Large open folds with NE – SW axes in this
Formation could be explained by local shortening due to
opposite sliding of the sediments of the hanging wall, top to
SW for the Puerta del Sol dome and top to NW for the
Mazatán Sierra dome (Fig. 8).
5. Local doming
Mazatán Sierra morphology is an isolated asymmetric
dome with a more abrupt eastern slope (Fig. 7). This
morphology is characteristic of some MCC, though some
display an elongation parallel to the maximum stretching
direction (Coney, 1979; Davis, 1980, 1983; Wernicke,
1985; Spencer and Reynolds, 1991). North of Mazatán
Sierra, another dome, Puerta del Sol Sierra (Fig. 8), is
limited along its western slope by mylonitic rocks in a
ductile, low-angle fault, which could be related to the
detachment fault of Mazatán Sierra. The domical morphology in Mazatán Sierra is reflected by the conic
distribution of mylonitic foliation, which dips subparallel
to the topography around the range (Fig. 3d) and thus
indicates that doming is younger than the formation of
mylonite along the detachment fault and is superimposed on
a preexisting ductile zone. Both domes are separated by a
lower, relatively flat area, which corresponds to the
syntectonic basin, where the Miocene Belleza Formation
Fig. 8. Mazatán Sierra and Puerta del Sol Sierra domes. (1) alluvial
deposits; (2) basaltic andesite, (3) Miocene–Pliocene conglomerates; (4)
belleza Formation; (5) two-mica granite; (6) granitic dykes; (7) Laramide
and older intrusive rocks; (8) Tarahumara Formation; (9) Paleozoic units;
(10) normal fault; (11) low-angle normal fault; (12) anticlinal axis; (13)
synclinal axis; (14) inferred contact.
R.V. Granillo, T. Calmus / Journal of South American Earth Sciences 16 (2003) 193–204
201
Fig. 9. Cross-section B–B0 from Fig. 1, illustrating successive sliding in the western slope of Mazatán Sierra. (1) Alluvial fan; (2) belleza Formation; (3)
Tarahumara formation; (4) two-mica tertiary granite; (5) paleozoic limestone; (6) middle proterozoic monzogranite with xenoliths of regional metamorphic
rocks; (7) low-angle normal faults; (8) shear sense in mylonitic rocks. All zones are extensively brecciated.
Several hypotheses have been postulated to explain the
dome shape of the MCC; discussions can be found
elsewhere (e.g. Lister and Davis, 1989; Dickinson, 1991).
Spencer (1984) and Spencer and Reynolds (1989) propose
that dome morphology is an isostatic response to the
tectonic removal of upper crust. The lithospheric model of
Wernicke (1985) has a similar consequence, but the MCC
axis does not coincide with the thinnest part of the crust.
Thinning depends mainly on the amount of extension and
results in the uplift of isotherms associated with warping of
the lower plate. The isostatic rise, which would warp the
lithosphere, must have occurred at a larger scale to include
the whole range of MCC in Sonora, from Magdalena MCC
to Mazatán Sierra. However, another cause is necessary to
explain the small area doming (, 12 km diameter), which
we call ‘local doming’. Buck (1988) proposes that domical
morphology could arise from flexural deformation along a
brittle normal fault that displaces the ductile shear zone (like
a large drag fold). In this case, the range must have an
elongated shape parallel to the fault (perpendicular to
extension direction), but the morphology of the Mazatán
and Sierra del Sol Sierras is quite circular (Fig. 8). Some
MCC in Arizona even have elongated shapes perpendicular
to the postulated normal faults (e.g. Harcuvar, Harquahala,
South Mountain). An alternative model to explain elongated
domes parallel to the extension has been developed by
Spencer and Reynolds (1991).
Concentric granitic dikes in Puerta del Sol Sierra are
similar to cone sheets, and many granitic dikes in Mazatán
Sierra, perpendicular to the extension direction, are similar
to those dated 33 Ma (Damon, pers. comm.). These dikes
suggest the existence at depth of magmatic bodies (with
laccolithic shape) that could heat and uplift the lower plate
of the MCC, warping the ductile shear zone (Lister and
Baldwin, 1993). The doming deforms the syntectonic basin
created by upper plate displacement along the shear zone
and creates the conspicuous distribution of clastic deposits
presented by the Belleza Formation, which encircle the west
flanks of both domes (Fig. 8). Unfortunately, we have not
had geophysical evidence for the proposed magmatism at
depth.
Domical uplift increased the dip of the detachment
fault, and upper plate fragments were raised and then slid
catastrophically over younger rocks previously displaced
along normal faults subsidiary to the detachment fault
(note the distribution of Paleozoic limestone in Fig. 1). An
example of these slides is showed in Fig. 9, where
Paleozoic limestone slid over volcanic rocks ascribed to
the upper Cretaceous – Paleocene Tarahumara Formation,
which in turn had slid over Tertiary two-mica granite. The
directions of the blocks’ movement are indicated by
striations on the mylonitic foliation planes and have a
radial pattern in the western half of the Sierra. Doming
was followed by high-angle normal faulting, which
transected the dome, as might have occurred along the
eastern slope of Mazatán Sierra.
An 18 Ma apatite fission-track (AFT) apparent age was
obtained for sample SM 16 (Table 1), located on the lower
plate west of Bachán ranch at 1210 m a.b.s.l. (Calmus et al.,
1998). The mean confined track length of 15.68 ^ 1.15 mm
Table 1
Fission-track data from sample SM 16
Location
Latitude
north
Longitude
west
z
rf ðNf Þ
£105 cm2
riðNiÞ
£105 cm2
Number of
counted
grains ðNÞ
PðX 2 Þ
(%)
rdðNdÞ
£105 cm2
Age (Ma)
^ 1s
Mean track
length
mm ^ 1s
SD
Number of
tracks
measured
Cañada
Bachán
3,219,650
576,875
339
7.45 (255)
14.8 (505)
16
90.87
2.18 (14379)
18.6 ^ 1.4
15.68 ^ 406
1.15
8
r; Fossil (f) and induced (i) track densities; N; number of fossil (f) and induced (i) tracks counted; P ðX 2 Þ; probability of obtaining a X 2 value for v degrees of
freedom, with v; number of counted grains 2 1; rd and Nd; track density for the fluence glass monitors and number of tracks counted; SD, standard deviation of
the confined track length distribution. Coordinates are given in universal transverse meridian (UTM) system. Sample location matches the star in Fig. 1
202
R.V. Granillo, T. Calmus / Journal of South American Earth Sciences 16 (2003) 193–204
Fig. 10. Paleoposition of sample Sm 16 at 18 Ma based on AFT data.
indicates a rapid cooling across the apatite partial annealing
zone (Fig. 10). Although this result is not very reliable,
because of the small number of measured tracks, the age is
consistent with many other AFT ages previously obtained in
the Basin and Range provinces. For example, the weighted
mean AFT age for the Catalina Mountain forerange is
20.1 ^ 1.0 Ma (Fayon et al., 1996), and AFT ages fall
between 21 and 14 Ma in the Harcuvar Mountains and
between 16 and 13 Ma in the Buckskin and Rawhide
Mountains (Foster et al., 1993). Assuming a slip rate of 4–
8 mm yr21 along the detachment fault (consistent with
average slip rates calculated for various detachment faults
of MCC of southwestern Arizona and southeastern California; Foster et al., 1993) and a dip angle of 208, the time
needed to cool through the apatite partial annealing zone
would be between 1.6 and 800 Ka, which corresponds to a
cooling rate of 31.25 – 62.5 8C Myr21. Rapid cooling related
to detachment faulting also has been recognized in other
MCC, such as the forerange of the Catalina Mountains
(Davy et al., 1989; Fayon et al., 1996).
6. Discussion and conclusions
Mazatán Sierra is the southernmost MCC of the western
Cordillera and presents very similar geological and
structural features to other MCC of northern Sonora and
southern Arizona (Keith et al., 1980; Nourse et al., 1994). It
was formed during a mid-Tertiary extensional event superposed on a thickened crust resulting from Laramide or preLaramide compressional events (Coney and Harms, 1984).
Based on structural and geological data, our results
document the following evolution for the Mazatán Sierra:
During the Early Oligocene, peraluminous granite was
emplaced along the interface of the metamorphic –plutonic
basement and overlying Paleozoic sedimentary rocks.
Dynamic metamorphism occurred along a thick, planar,
gently dipping shear zone and produced a mylonitic fabric at
depth. In the upper plate, tilting of the faulted blocks
allowed for the formation of a basin at the surface, which
was filled with fanglomerates, sandstones, lacustrine
deposits, and a few interbedded volcanic rocks. The position
of the sediments in the hanging wall of the MCC, their
lithology, the variation of dip, faulting, and slumping coeval
with sedimentation indicate a syntectonic character of the
Belleza Formation. Finally, upward warping in response to
intrusive bodies and tectonic denudation above the shear
zone bent the mylonitic foliation of the lower plate and may
have accelerated the unroofing. Brittle deformation was
superposed over the mylonitic shear zone. Granitic dikes
that cross-cut the Mazatán Sierra suggest that the arching
may be partly due to intrusions below the core of the Sierra.
Doming caused sliding of blocks composed of Paleozoic
sedimentary rocks that belong to the upper plate, particularly along the western flank.
We suggest that the Mazatán Sierra MCC belongs to a
larger normal shear zone that includes Puerta del Sol Sierra
and the area south of the Aconchi batholith. Nourse et al.
(1994) map middle Tertiary mylonitic fabric west of Puerta
del Sol Sierra. The mylonitic shear zone is hidden between
the Sierras, where the Belleza Formation was deposited.
Probably only one shear zone extends along Mazatán Sierra
through Magdalena Sierra, which would be discontinuously
exposed by local doming.
Acknowledgements
We are indebted to Marc Sosson, who reviewed a
previous version of this paper, and Sheri Kelley and Jon
Spencer, who reviewed the submitted version. We thank
also Erika Labrin, who measured the length of the fission
R.V. Granillo, T. Calmus / Journal of South American Earth Sciences 16 (2003) 193–204
tracks. We are grateful to Pablo Peñaflor-Escarcega at the
Estación Regional del Noroeste, Hermosillo, François
Senebier and Francis Coeur at the Institut Dolomieu,
university Joseph Fourier, Grenoble, who were in charge
of sample preparation.
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