Download The role of crustal heterogeneity in controlling vertical coupling

The role of crustal heterogeneity in controlling vertical coupling
during Laramide shortening and the development of the
Caribbean –North America transform boundary in southern
Mexico: insights from analogue models
MARIANO CERCA1, LUCA FERRARI1, MARCO BONINI2,
GIACOMO CORTI3 & PIERO MANETTI4
1
Centro de Geociencias, Universidad Nacional Autónoma de Mexico, Campus Juriquilla,
Apartado Postal 1-742, Querétaro 76230, Mexico
(e-mail: [email protected], [email protected])
2
Consiglio Nazionale delle Ricerche, Istituto di Geoscienze e Georisorse,
Sezione di Firenze, via G. La Pira 4, 50121 Firenze, Italy
3
Dipartimento di Scienze della Terra, Università degli Studi di Pisa,
via S. Maria 53, 56126 Pisa, Italy
4
Consiglio Nazionale delle Ricerche, Istituto di Geoscienze e Georisorse,
via G. Moruzzi 1, 56124 Pisa, Italy
Abstract: Analogue models of polyphase deformation involving crustal differences in
strength, thickness and density give insights into lateral and vertical strain propagation
during Late Cretaceous shortening and Early Tertiary left-lateral shearing related to the
early development of the North America– Caribbean plate boundary in southern Mexico.
Analogue models reproduce a two-phase deformation characterized by a first stage of
compression orthogonal to the plate boundary, simulating deformation induced by the
Laramide orogeny, followed by a later stage of left-lateral transpression associated with
the transfer of the Chortis block from the North American to the Caribbean plate during
the early stage of development of the new plate boundary in Early Tertiary times. Based
on detailed structural observations in the Guerrero – Morelos platform and the western
part of the Mixteco terrane of southern Mexico, we document that a transpressive regime
affected continental red bed sequences of Early Paleocene to Late Eocene, and rotated
and refolded Laramide structures during this second phase. Our model ends before the
transtensional regime that affected the region, which is marked by a volcanic episode of
Late Eocene –Oligocene. This change in the deformation regime records the passage of
the NW tip of the Chortis block (North America – Cocos– Caribbean triple junction),
when subduction replaced transform faulting along the southern Mexico margin. The
models focus on the structures formed around the flanks of a thicker/more rigid crustal
block that simulates the rock assemblages of the Palaeozoic orogens of southern Mexico
(Mixteco – Oaxaca –Juarez block, MOJB). The comparison of the mechanism of deformation of three different analogue models with the natural prototype explains most of
the structures observed around the MOJB. Counterclockwise vertical-axis rotations of
pre-existing structures in the western flank of the MOJB observed in the Guerrero –
Morelos platform are consistent with the modelled structures. Vertical movements of the
modelled MOJB induced by the transpressive regime can explain the Papalutla thrust and
the basement upheaval and gravitational sliding of the cover in the Tentzo Ranges observed
at the western and northern margins of the MOJB, respectively. The growth and propagation
of thrusting controlled by the geometry of the block along the eastern margin also correlates
with the Vista Hermosa fault. The propagation of strain to the north increases with higher
contrast in strength of the thick block with respect to the adjacent modelled crust. Analogue
modelling failed to reproduce all the structural details of southern Mexico and, specifically,
the structures observed inside the MOJB. The latter, however, are controlled by pre-existing
discontinuities, which are not simulated in the model. As a whole, the results demonstrate
that crustal heterogeneity in a developing left-lateral plate boundary zone produces a
stronger vertical coupling between ductile and brittle crust and a widening of the deformation zone along the margin of the North America plate in southern Mexico.
From: GROCOTT , J., TIKOFF , B., MC CAFFREY , K. J. W. & TAYLOR , G. (eds) 2004. Vertical Coupling and
Decoupling in the Lithosphere. Geological Society, London, Special Publications, 227, 117–140.
0305-8719/04/$15 # The Geological Society of London 2004.
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M. CERCA ET AL.
Unlike convergent margins, deformation along
transform plate boundaries on continental lithosphere can affect a relatively narrow zone on
both sides of the plates (e.g. North Anatolia
fault, Polochic–Motagua fault system). However,
it can be argued that, during the initial development of a transform boundary, deformation is
most commonly accommodated in a wide zone,
whereas in a mature stage strain localizes along
discrete systems of lithospheric strike-slip faults
(e.g. Gordon 1998). The degree of coupling
between the upper mantle, the lower crust and
the upper crust ultimately controls the width of
the deformation zone at the early stage of development of the boundary. Furthermore, the presence
of crustal blocks with different thicknesses and
strengths is likely to alter the coupling and to
control the transmission of deformation toward
more internal zones of the plates.
The development of the North America –
Caribbean plate boundary in the Early Tertiary
can help to elucidate the importance of crustal
heterogeneity during the early development of
a transform boundary. It has been suggested
that the Chortis block of Central America was
an integral part of the North America plate from
Jurassic to Late Cretaceous times (Meschede &
Frisch 1998, and references therein). At the end
of Late Cretaceous, the Chortis block began to
detach from North America and to move eastward with the Caribbean plate, likely as a consequence of a reorientation from normal to oblique
subduction of the Farallon plate (Herrmann et al.
1994; Meschede et al. 1996). As a result, the
continental margin of southern Mexico was
truncated (Riller et al. 1992; Herrmann et al.
1994; Schaaf et al. 1995) and middle to lower
crustal rocks (Xolapa complex) were exhumed
along a 60 km wide band to the north of new
plate boundary (Morán-Zenteno et al. 1996).
Non-coaxial deformation and migmatization
were inferred to have developed in the Xolapa
complex between 70 and 46 Ma (Herrmann
et al. 1994; Meschede et al. 1996) or in Early
Cretaceous (Morán-Zenteno 1992; Ducea et al.
2003). Mylonitic zones developed in a general
left-lateral transtensional regime are observed
bounding middle to lower crustal rocks of the
Xolapa complex to the north (Fig. 1).
Vertical propagation of strain in the upper
crust and horizontal transmission to the north
of the shear zone that bounds the Xolapa complex are not well understood. Meschede et al.
(1996) used inversion of brittle microstructures
Fig. 1. Terrane boundaries (thick dashed grey lines) and major structural features of southern Mexico (modified after
Campa & Coney 1983; Sedlock et al. 1993). TMVB, trans-Mexican volcanic belt. Terranes: G, Guerrero; M, Mixteco;
O, Oaxaca; J, Juarez; Ma, Maya; and X, Xolapa. GMP, Cretaceous Guerrero–Morelos platform. Structures: (1)
Teloloapan– Ixtapan de la Sal thrust; (2) Zitlala–Cuernavaca thrust; (3) refolded and rotated Laramide folds; (4)
Papalutla thrust; (5) Tentzo Ranges arcuate folds; (6) Oaxaca fault; and (7) Vista Hermosa thrust. Along the northern
boundary of the Xolapa terrane, several mylonitic zones (Mz) crop out in the Tierra Colorada shear zone (TC), and
Chacalapa fault (Ch). Black dashed lines show the approximate boundaries of the Mixteco–Oaxaca–Juarez block
(MOJB) and black solid lines mark thrust boundaries.
COUPLING DURING LARAMIDE SHORTENING
measured in a wide area of southern Mexico to
claim that the stress applied at the plate boundary
has been transmitted to the north of the Xolapa
complex into the Mixteco and Oaxaca terranes
(Fig. 2). They grouped structures in Palaeozoic
to Early Tertiary rocks inferred to have developed between 70 and 40 Ma in a unique event
characterized by subhorizontal s1 and s3, i.e. a
left-lateral strike-slip regime of deformation.
Recently, however, we have documented a
more complex deformation history. Detailed
field studies in the Guerrero –Morelos platform
and the eastern part of the Mixteco terrane
(Fig. 1) show that a major episode of east–west
119
shortening between 88 and 67 Ma (Laramide
orogeny) was followed by Early Tertiary leftlateral transpression that affected an area up
to 250 km to the north of the modern plate
boundary (Cerca & Ferrari 2001). Starting from
Latest Eocene, transpression was replaced by
transtension, which triggered widespread silicic
volcanism (Morán-Zenteno et al. 1999; AlanizAlvarez et al. 2002). The Early Tertiary transpression was particularly diffuse at the boundary
and in the eastern part of the Mixteco–Oaxaca –
Juarez block (MOJB), a thicker and more rigid
crustal block, suggesting that it controlled the
widening of the deformation zone related to the
Fig. 2. Sketch map showing the main lithological units of southern Mexico (modified after Ortega-Gutiérrez et al.
1992; Consejo de Recursos Minerales 2001). GC, Guichicovi complex. Solid lines indicate major fault zones.
Q9
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M. CERCA ET AL.
development of the Caribbean –North America
plate boundary.
In this Chapter we approach the study of deformational features around the MOJB indirectly,
by performing analogue models designed to
investigate the space–time propagation of
deformation in relation to crustal rheological
heterogeneities during polyphase deformation
simulating the tectonic evolution of the southern
Mexico deformed margin.
Geological and tectonic setting
Crustal structure of southern Mexico and the
Mixteco –Oaxaca – Juarez block
Geologically, Mexico south of the trans-Mexican
volcanic belt (TMVB) consists of a heterogeneous mosaic of crustal blocks (Fig. 1) that
have been traditionally classified using the
tectonostratigraphic terrane analysis (Campa &
Coney 1983; Sedlock et al. 1993). Owing to
their differing geological histories, these crustal
blocks or terranes have distinct thicknesses and
rheologies that must be considered in any deformation modelling. The Mixteco and Oaxaca
terranes are considered to have the oldest basement in southern Mexico. These terranes are
mainly composed of Precambrian or Palaeozoic
metamorphic rocks, and a Jurassic to Early
Tertiary sedimentary and volcanic cover (Campa
& Coney 1983; Sedlock et al. 1993). The
Mixteco terrane records a Late Ordovician–
Early Silurian continental collisional event
(Acatecan orogeny) related to the closure of the
Iapetus Ocean (Ortega-Gutiérrez et al. 1999)
and was later sutured to the Grenvillian Oaxaca
terrane in the Early Permian (Elı́as-Herrera &
Ortega-Gutiérrez 2002).
The basement of the Juarez terrane is poorly
known, but recent studies suggest that it could
be also pre-Mesozoic. The boundary between
the Oaxaca terrane and the Juarez terrane is the
north–south-trending Sierra de Juarez mylonitic
complex, which records right-lateral movements
related to the southward movement of the
Yucatan block and the opening of the Gulf of
Mexico in Mid-Jurassic times (Alaniz-Alvarez
et al. 1996) (Fig. 1). It has also been suggested
that the Juarez terrane was the site of a rifting
in Jurassic times (Sedlock et al. 1993, and references therein). However, Jurassic volcanism is
very limited and recently published regional
maps show a wide area of Palaeozoic metamorphic rocks in the core of the Juarez terrane
(Consejo de Recursos Minerales 1998, 2001).
In addition, the protolith of the Sierra de Juarez
mylonites is, at least in part, the Grenvillian
Oaxaca terrane (Alaniz-Alvarez et al. 1996),
and Grenvillian rocks have been reported in the
Guichicovi complex SW of the Juarez terrane
(Murillo et al. 1992; Weber & Köhler 1999)
(Fig. 2), which suggests a lateral continuity of
the rocks in between. This seems to be confirmed
by a magnetotelluric study crossing the Oaxaca
and Juarez terranes, which indicates that they
could share a similar basement at depth
(Jording et al. 2000). The eastern boundary of
the Juarez terrane is the Vista Hermosa fault
zone (Ortega-Gutiérrez et al. 1990), east of
which is the transitional crust of the Maya
terrane thinned during the opening of the Gulf
of Mexico (Fig. 1).
Given its geological characteristics, we consider that the block (MOJB) formed by the
Mixteco, Oaxaca and Juarez terranes presents a
more rigid basement and a thicker crust than
the surrounding regions. To the west of this
block, Mesozoic island arc assemblages and
Cretaceous marine carbonates crop out extensively (Guerrero terrane and Guerrero–Morelos
platform, Figs 1 & 2) and the crust is relatively
thinner. The western limit of the block corresponds to the contact between the deformed
and metamorphosed basement (Acatlán complex)
of the Mixteco terrane and the Cretaceous
carbonate sequences of the Guerrero –Morelos
platform. Finally, further geometrical constraints
are provided by the presence of a semiarc-shaped fold-and-thrust belt that surrounds and
follows approximately the border of the MOJB
(Figs 1 & 2).
Available geophysical data indicate that the
crustal thickness decreases from 45 km in the
central part of the MOJB to 28 km north of
Zihuatanejo and to 25 km in the Tehuantepec
Isthmus zone (Campos-Enriquez & SanchezZamora 2000; Valdéz et al. 1986; UrrutiaFucugauchi & Flores-Ruiz 1996; Garcı́a-Pérez
& Urrutia-Fucugauchi 1997). Thinning or absence
of the Late Cretaceous carbonate sequences in
the Mixteco, Oaxaca and Juarez terranes indicates that the MOJB was at least partially emergent and represented a major heterogeneity of
the southern Mexico crust with well-developed
boundaries by Late Cretaceous times (Fig. 3).
Laramide deformation
After earlier deformation phases, southern Mexico
was affected in Late Cretaceous time by contractional deformation during the Laramide orogeny
(Lang et al. 1996; Bird 1998; Cabral-Cano et al.
2000a). The migration of deformation towards the
continent has been associated to low-angle and
high-velocity subduction in the western margin of
COUPLING DURING LARAMIDE SHORTENING
121
Fig. 3. (a) Digital elevation model of the southern Mexico continental margin and (b) idealized longitudinal
section. Analogue models were designed to investigate the influence of a rigid block in the brittle crust. In the
models the section was simplified assuming a stratified two-layer and uniformly thick crust.
the North America plate (Bunge & Grand 2000).
The onset of the Laramide deformation in southern
Mexico has been constrained by the deposition of
the Mezcala flysch, which records a sudden
change from carbonaceous to terrigenous sedimentation and represents the youngest deformed unit.
This transition is set at the Cenomanian–Turonian
boundary (c. 93 Ma) (Hernández-Romano et al.
1997) or at the Turonian–Coniacian boundary (c.
89 Ma) (Lang & Frerichs 1998). Almost all
authors agree in placing the end of the Laramide
episode in the Paleocene in view of time constraints
north of the TMVB (Salinas-Prieto et al. 2000, and
references therein). However, volcanic and plutonic
rocks of 67–62 Ma unaffected by Laramide-style
deformation suggest a Maastrichtian age at least
in the Guerrero–Morelos platform area (OrtegaGutiérrez 1980; Meza-Figueroa et al. 2001).
The Laramide orogeny produced regional
ENE-directed shortening that presumably amalgamated and stacked the volcanic arcs and sedimentary successions of the Guerrero terrane onto
an attenuated continental crust (Cabral-Cano
et al. 2000a, b). The result of the shortening is
manifested in a wide north –south-striking
fold-and-thrust belt with vergence towards the
ENE. According to Salinas-Prieto et al. (2000),
progressive shortening caused a second ductile
deformation with opposite vergence (backthrusting) of structures.
Early Tertiary deformation
The rock sequences recording the time interval
between Maastrichtian and Late Eocene in the
study area consist mainly of continental sedimentary deposits and minor volcanic rocks (e.g.
Tetelcingo, Balsas, Oapan and other locally
named formations) that fill basins bounded by
north–south folds and thrusts formed during previous eastward shortening. Until recently, shortening structures of contrasting style affecting
these Early Tertiary sequences have been attributed to the Laramide deformation. Complex patterns of shortening and associated strike-slip
faults have been observed widely in these
sequences north of the shear zone bounding the
Xolapa complex. In particular, deformation
decreasing gradually upwards can be observed
in the continental deposits of the Balsas and
Tetelcingo formations on the western flank of
the MOJB (Fries 1960; De Cserna et al. 1980;
Ortega-Gutiérrez 1980) (Fig. 4). The most
notable example is a wide deformation band
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M. CERCA ET AL.
Fig. 4. Simplified geology of the western margin of the MOJB (Mixteco terrane) and Guerrero–Morelos platform
and location of the Early Tertiary basins deformed.
(60 km) in front of the NE–SW-trending
Papalutla thrust that affects Tertiary volcanosedimentary deposits of the Copalillo and Tuzantlan
basins (Fig. 4). Deformation, characterized by
NW-directed shortening and NW–SE strikeslip faults, is more intense near the Papalutla
fault (Cerca & Ferrari 2001). This structure
dips to the east and along 9 km of its length
thrusts Palaeozoic rocks of the Mixteco terrane
on top of the Cretaceous sedimentary succession
of the Guerrero–Morelos platform. Models that
characterize Laramide deformation by eastverging thrusting of the Late Cretaceous
sequences (Campa 1978; Campa & Coney
1983; Salinas-Prieto et al. 2000) fail to explain
the geometry of this fault. On the other hand,
our recognition of a deformation consistent
with the geometry of the Papalutla fault in the
Copalillo and Tuzantlan continental deposits
(Figs 4 & 5) suggests that it moved during the
Early Tertiary, although previous movements
are not discarded.
In the middle of the Guerrero –Morelos platform, where the thickness of the sequences in
the Balsas basin reaches c. 500 m, deformation
characterized by a large NW– SE fold and the
absence of normal faults is worth nothing. The
intensity of folding decreases towards the top
of the sequence (Fig. 5a –c).
Towards the southern part of the Guerrero –
Morelos platform, deformation is characterized
by asymmetric NW –SE synclinorium-type
structures (Chilpancingo basin) and NW –SE
strike-slip faults (Fig. 4). Near the boundary
with the Xolapa complex, the Late Cretaceous
carbonates are refolded and aligned with east –
west-trending folds with vertical hinge lines
(Fig. 5d). In this area, large outcrops of carbonate
COUPLING DURING LARAMIDE SHORTENING
123
Fig. 5. (a) Panoramic view to the NE of the Balsas sequence in the central Guerrero–Morelos platform showing
change in the inclination of layers. (b) From bottom to top, Maastrichtian Tetelcingo, Paleocene Balsas and Oapan
formations, the sequence is covered by undeformed Oligocene volcanic rocks. (c) Deformation of a volcano-sedimentary
sequence in the Tertiary basins in front of the Papalutla thrust. (d) View to the west in the Mexico–Acapulco
highway showing Cretaceous limestone of Paleocene. (e) Vertical bedding of Paleocene conglomerate within a tight
fold in the Yanhuitlan sequence.
breccias adjacent to the mylonites and underlying the Early Tertiary sequences are evidence
of the rupture and detachment of the Chortis
block (Mills 1998).
Within the MOJB, localized folding associated
with strike-slip movements of the same age has
been observed in the Yanhuitlan area (Fig. 5e).
Other structures (mostly strike-slip faults and
minor folds) within and east of the MOJB have suspected Paleocene to Eocene age and affect the plate
margin in an area of variable width to the north of
the shear zone bounding the Xolapa complex.
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M. CERCA ET AL.
Tertiary deformation
All the above evidence indicates that, in the
Guerrero–Morelos platform and the western part
of the MOJB, deformation during Early Tertiary
times was distinct from the Late Cretaceous
Laramide shortening and was essentially transpressional. Constraining Laramide shortening
between the time interval from 88 to 67 Ma
implies that in southern Mexico it commenced
earlier and continued for a shorter time than in
the north (75–40 Ma; Bird 1998, and references
therein). This difference suggests that a change
in the tectonic setting of southern Mexico occurred
during the Late Maastrichtian–Early Paleocene
interval. There is ample evidence that an ESE
left-lateral strike-slip regime over a broad zone
dominated the Cenozoic tectonics of the MOJB.
This regime has been related to the detachment
of the Chortis block from North America and its
transfer to the eastward-moving Caribbean plate.
This process is likely to have begun at the end of
the Late Cretaceous as a consequence of changes
in the angles of convergence and subduction
between the Farallon and North America plates
(Engebretson et al. 1985; Ratschbacher et al.
1991; Herrmann et al. 1994; Meschede et al.
1996). A second important change occurred
when the northwestern tip of the Chortis block
and the trench–trench–transform triple junction
passed along the coast of southern Mexico.
According to the model of Morán-Zenteno et al.
(1996), the uplift of the continental margin
and the exhumation of the middle crustal rocks
of the Xolapa complex followed the passage of
the triple junction. Uplift and exhumation were
accomplished through the development of the
mylonitic zone bounding the Xolapa complex to
the north. Available isotopic ages and crosscutting
relationships between plutons and the mylonitic
zones also indicate that magmatism was active
just before and after the triple junction passage
(Schaaf et al. 1995; Morán-Zenteno et al. 1999).
In the area of the Guerrero– Morelos platform,
the triple junction passage is well documented by
a widespread Late Eocene –Early Oligocene
(36–30 Ma) volcanic episode (Morán-Zenteno
et al. 1999, and references therein; Cerca et al.
2003). In the northern part of the Guerrero –
Morelos platform, this last volcanic episode has
been associated with a transtensional regime
(Alaniz-Alvarez et al. 2002). Post-Eocene transcurrent and transtensional deformation is widespread also to the east. Indeed, Meschede et al.
(1996) obtained a strike-slip palaeotensor from
faults in volcanic rocks at Chilapa (their site
CHI2-S) that we dated at 32.7 Ma (Cerca et al.
2003), and a transtensional palaeotensor by
inversion from faults in the Etla tuff (site ETVS2) dated at 17 Ma (Urrutia-Fucugauchi &
Ferrusquı́a-Villafranca 2001).
Summarizing all the above information, we
propose that the Early Tertiary strain
(65–36 Ma) constituted a phase of deformation
different from the Late Cretaceous Laramide
shortening and the post-Eocene transtension.
This deformation is represented overall by localized tectonic dragging effects and small counterclockwise rotations about the vertical axis of
previously formed structures and semi-rigid
crustal blocks. Consistent counterclockwise
rotation of Laramide structures and Early Tertiary
sequences (Balsas formation) has also been found
in palaeomagnetic studies (Molina-Garza et al.
2003, and references therein). We propose that a
transpressive regime also affected the study
region during these times based on the following
considerations: (a) consistent folding and strikeslip faults are observed in the Early Tertiary red
bed sequences that record the time interval
between the Early Paleocene and the Late
Eocene, this deformation decreasing gradually to
the top of the sequence; (b) there is a remarkable
absence of major normal faults affecting these
sequences; and (c) north–south-trending, vertical
hinges of Laramide folds are refolded as a consequence of the strike-slip of lower crust rocks.
In this context we hypothesize that this heterogeneous Early Tertiary deformation can be
ascribed to a general left-lateral strike-slip regime
at the early stages of the Chortis block transfer to
the Caribbean plate (Fig. 6). As mentioned
above, this deformation regime was triggered by
changes in convergence direction and subduction
angles between the Farallon and North America
plates. Crustal strain was distributed in a wide
area along the developing transform plate boundary. The presence of the thicker and more rigid
MOJB caused an inland propagation of deformation within a transpressional regime. This
deformation decreased gradually as it was accommodated heterogeneously by rotation of structures
and newly formed discrete shear zones. With the
passage of the trench–trench–transform triple
junction in the Late Eocene–Oligocene, the transform boundary was replaced by subduction. This
represents a free boundary that triggered transtension inside the continental margin.
Analogue modelling of the Late Cretaceous
and Early Tertiary deformation
Model construction
Experiments were performed at the Tectonic and
Geomorphic Processes Modelling Laboratory of
COUPLING DURING LARAMIDE SHORTENING
125
Fig. 6. Cartoon showing the hypothetical model of the deformation phases. (a) Laramide deformation during the Late
Cretaceous caused a wide fold-and-thrust belt. (b) During Early Tertiary transpression, new structures form around the
MOJB, counterclockwise rotation and Laramide structures are refolded, rotation reaches c. 158. Nomenclature as in
Figure 1, plus Yu, Yucatan block. The position of the later exhumed Xolapa terrane is indicated (X). Arrows indicate
approximate vector of convergence between Farallon and North America plates, after Engebretson et al. (1985),
Meschede & Frisch (1998) and Bunge & Grand (2000).
the CNR-IGG at the University of Florence,
Italy, and were built in a ‘squeeze box’ type
apparatus (Fig. 7). The apparatus consists of a
metallic table with a fixed wall on one side. On
the opposite side there is a parallel wall that is
allowed to move in different directions. Displacement of this moving wall, which is produced
by electric motors and controlled (in terms of
direction and velocity) by a central unit, allows
simulation of normal and oblique extension,
orthogonal to transpressive contraction and
strike-slip deformation.
Models, with dimensions of 40 cm length,
39 cm width and 1.55 cm average thickness,
were built on the metallic table of the experimental apparatus, between the fixed and moving
walls (Figs 7 & 8a). Experiments were designed
to reproduce a two-phase convergence: a first
phase of orthogonal compression followed by a
later stage of shortening with a lateral component
in a NE direction. In order to obtain orthogonal
convergence during the first phase, the moving
wall of the experimental apparatus was modified
by fixing an orthogonal short metallic wall at one
of its extremities and producing an L-shaped wall
that was allowed to move in a parallel direction
with respect to the fixed wall. In this setting,
the short metallic wall produced orthogonal
compression of the models, with a direction of
shortening that was parallel to the fixed metallic
wall. The models were shortened by 42 mm
(11% bulk shortening) at a velocity of 6 mm h21
during this phase. During the second phase, the
short metallic wall was removed and the
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M. CERCA ET AL.
Fig. 7. Motorized analogue modelling apparatus used to perform the experiments.
moving wall was displaced in such a way as to
create transpression at an angle of 158 with
respect to the fixed wall. Models were shortened
by 72 mm (17% bulk oblique shortening) at a
velocity of 15 mm h21 during this second
phase. Three representative models are discussed
in detail in this work (see Table 1). Photographs
were taken at regular time intervals with vertical
and lateral illumination to observe the development and propagation of the structures. At the
end of each experiment, models were covered
by white sand to preserve the final topography
and subsequently soaked in water to allow nondisturbed cuts of longitudinal sections.
Model rheological structure and analogue
materials
We constructed models that were designed to
simulate a simplified two-layer vertical rheology:
a brittle upper crust and a lower ductile crust
(Fig. 8c & f). A parallelepiped-shaped built-in
block representing the rigid block was constructed in one side of the models, adjacent to
the moving wall (Fig. 8b). The parallelepiped
was 25 cm long and 15 cm wide. The right
and left external sides of the parallelepiped had
angles of 458 and 358 with respect to the
moving wall. The thickness of the brittle material
was 7.5 mm, but increased to 11.5 mm within the
rigid block.
Dry quartz sand with well-rounded and uniform
size grains (0.24 mm) was used to model the
brittle behaviour of the upper crust. Quartz sand
has a mean density of 1400 kg m23 and insignificant cohesion (70 Pa). Layers of coloured sand
were sieved and sedimented as passive markers to
highlight deformation in the longitudinal sections.
In models Chortis 02 and 03, the rigid block
was simulated by thickness variation of the sand
layer in order to create lateral strength heterogeneity. In model Chortis 04, the rigid block
was simulated by using humid plastic clay to
enhance the strength contrast between the block
and ‘normal crust’ and to exaggerate its influence
on the model deformation pattern. Plastic clay
has a mean density of 2500 kg m23 and unscaled
high cohesion compared with the sand. A rough
estimation of the clay cohesion using a shear
vane tester yielded values ranging between c.
37 and 59 kPa. Model Chortis 03 was varied
slightly from model Chortis 02 by lubricating
the metallic table with Vaseline oil in order to
reduce basal friction.
To simulate the ductile behaviour of the lower
crust, a homogeneous mixture of silicone polymer
(Mastic Silicon Rebondissant No. 29 provided by
CRC Industries, France) and sand (with a silicon:
sand ratio of 5:5.5 by weight) has been utilized.
The mixture has a red colour, a density ranging
from 1450 to 1500 kg m23, and a dynamic
shear viscosity h ¼ 3 105 Pa (determined at
218C using a coni-cylindrical viscometer).
COUPLING DURING LARAMIDE SHORTENING
127
Fig. 8. Construction of the model: (a) 3D view; (b) map view; and (c) longitudinal section of models Chortis 02 and 03
and location of profiles. Strength profiles: (d) the modelled crust, (e) the modelled block constructed with sand,
(f) longitudinal section of model Chortis 04, and (g) the modelled block constructed with clay.
This material exhibits Newtonian behaviour at
low strain rates such as those occurring in the
experiments (,1023 s21). After construction, a
grid of sand over the model surface served as a
passive strain marker for the map views. The
initial strength profiles of the models for
maximum values of strain rate (1̇ 1023 s21)
produced in the second phase are presented in
Figure 8d, e and g. The properties of the materials
used and a comparison with the natural properties
are summarized in Table 2.
Scaling of models
Scaling of a model is based on the kinematics,
dynamics and geometrical similarities between
the model and the natural prototype (Hubbert
1937; Ramberg 1981). Models were designed
specifically to simulate the two phases of deformation that affected the south of Mexico. Thus,
simplified geometry and boundary conditions
were reproduced using all the geological, structural, geochronological, and geophysical information available for southern Mexico.
128
M. CERCA ET AL.
Table 1. Characteristics of the three experiments
Chortis 02
Material used for the thicker block
Cohesion contrast of the block with
respect to the adjacent upper crust
Lubrication at the base of model
Nature of initial boundary between
thicker block and lower crust
Chortis 03
Chortis 04
Sand
No
Sand
No
Humid clay
High
No
Vertical
Vaseline oil
Vertical
No
Vertical
shear viscosity of the silicon–sand mixture of
3 105 Pa s, then h ¼ 3 10217 and 1̇ ¼
8.5 109. The horizontal displacement velocity
can be calculated from v ¼ vmodel/vnature ¼
1̇ l , giving v ¼ 4250.
In the first phase of deformation, experiments
represented ENE progressive Laramide shortening
active during the interval from Turonian to
Maastrichtian –Earliest Paleocene. Actual peak
velocities computed for the Colorado Plateau in
the Laramide orogeny of the Rocky Mountains
are 1.5 mm a21 (Bird 1998). However, in the
Rocky Mountains the Laramide orogeny occurred within a continental plate, not involving
large deformations or displacements and in a
different period of time from 75 to 35 Ma (Bird
1998). In southern Mexico, the Laramide is a
progressive deformation directed to the east
that affects a wide area and crustal blocks of
diverse composition (Salinas-Prieto et al. 2000;
Cabral-Cano et al. 2000b). Laramide shortening
in southern Mexico was estimated at approximately 60 km on a balanced section in the
eastern Guerrero terrane (Lang et al. 1996) and
Models were properly scaled to Nature in such
a way that 1 cm in the model is equivalent to
20 km in Nature and geometric similarity l ¼
lmodel/lnature ¼ 5 1027. In the same way, the
normal stress ratio between the model and
Nature must be scaled with the general stress
reduction equation s ¼ smodel/snature ¼ r g l ,
where the asterisks represent the ratio of the
variable in the model and in Nature. This equation can be reduced to s ¼ r l , because tests
on the models were conducted under normal
gravity conditions, g ¼ 1. The mean value of
density in the brittle upper crust is approximately
2750 kg cm23, and the ratio r ¼ 0.51, so the
density ratio between brittle and ductile crust
(BC/DC) is approximately 0.95. With these
values, a stress ratio of s ¼ 2.55 1027
between model and Nature has been calculated.
In ductile materials, viscous forces are related
to dynamic shear viscosity and strain rate by
s ¼ 1̇ h or 1̇ ¼ s /h . Assuming a reasonable
value for the dynamic shear viscosity of the
Q1 lower crust of 1021 to 1023 Pa s (Corti et al.
2002; Willner et al. 2002), and for the dynamic
Table 2. Model and natural parameters used in experiments
Parameter
Density, BC (kg cm23)
Cohesion, BC (kPa)
Coefficient of friction, BC, m
Density, DC (kg cm23)
Viscosity, DC, n (Pa s)
Gravity, g (m s22)
Length, l (m)
Stress, s (Pa)
Strain rate, 1̇ (s21)
Time, 1st phase, t (s)
Time, 2nd phase, t (s)
Velocity of displacement,
1st phase, v (m s21)
Velocity of displacement,
2nd phase, v (m s21)
Chortis 02 and
03
Chortis 04 clay
block
Nature
Model/nature ratio
(referred to models
Chortis 02 and 03)
1400
insignificant
0.7002
1450
3 105
9.81
0.01
2500
37–59†
0.51
9.81
0.01
2750
6 107
0.6 – 0.85
2900
1021 –1023
9.81
20000
2.82 104
1.72 104
1.67 1026
2.3 10214
6.62 1014
8.19 1014
3.93 10210
0.5
3 10217
1
5 1027
2.55 1027
8.5 109
3.80 10211
2.11 10211
4.25 103
4.17 1026
9.83 10210
4.25 103
2 1024
2.82 104
1.72 104
1.67 1026
(6 mm h21)
4.17 1026
(15 mm h21)
BC, brittle crust (sand); DC, ductile crust (silicon – sand mixture).
Estimated at 24% water content with a vane tester.
†
COUPLING DURING LARAMIDE SHORTENING
available age constraints indicate that it occurred
in about 20 Ma. Using these values we obtain a
rough estimation of velocity of 3 mm a21,
twice the velocity in the Rocky Mountains, and
vmodel of 1.5 mm h21. In order to carry out the
models in a convenient experimental time, we
assumed a vmodel ¼ 6 mm h21. With this setup
the experimental time decreased without a significant change in the resulting structural pattern.
The second phase of deformation simulated the
left-lateral transpressive regime affecting the area
between the Early Paleocene and Late Eocene.
Meschede & Frisch (1998) estimated over
1000 km of displacement of the Chortis block
during the Palaeogene. Previous studies calculated
deformation velocities between 54 and 56 mm a21
(Herrmann et al. 1994; Schaaf et al. 1995) assuming a narrow plate boundary where strain is
accommodated in localized shear zones. However,
in a diffuse plate boundary, velocities can decrease
considerably because strain is accommodated in a
wide area (Gordon 1998). Furthermore, the rheological heterogeneities and mechanical anisotropy
of the continental margin play an important role in
the propagation and partitioning of deformation
during orogenic events (Vauchez et al. 1998).
With these values, a reasonable deformation
velocity of approximately 32 mm a21 was assumed and a model velocity of 15 mm h21 in an
ENE direction was computed.
Model results
Evolution of deformation during the first phase
Map views and line drawings showing the evolution of deformation during the first phase of
deformation are presented in Figure 9. At 3%
bulk shortening (3% b.s. ¼ 12 mm), the initial
model deformation is manifested at the surface
as north –south-striking thrusts, with vergence
toward the foreland, developed at about 3.5 to
4 cm in front of the moving wall (Fig. 9b, & j).
At 9% b.s., a second thrust was created 4 cm in
front of the first structure in the models Chortis
02 and Chortis 04 (Fig. 9c & k). In the model
Chortis 03, the second thrust with the same
vergence was formed earlier at about 4.5% b.s.,
4 cm in front of the first structure. In this
model, at 9% b.s. several discontinuous folds
and a thrust formed between the two main structures, up to 3 cm in front of them (Fig. 9g).
The models show differences in the deformation at the end of the first deformation
phase, at 10.5% b.s. A third thrust with a
regular interval of 4 cm was formed in front of
the second structure in the model Chortis 02
(Fig. 9d). This structure followed the shape of
129
the rigid block without affecting it significantly.
In the models Chortis 03 and Chortis 04, increasing displacements along the existing structures
were registered (Fig. 9h & l) but no new structures were formed. A general characteristic of
the deformation during the first phase is the periodic growth of the main thrusts with a spacing of
3.5–4 cm. The model Chortis 03 also developed small backthrusts, likely as a consequence
of decreasing the friction by lubricating the
base with Vaseline oil.
It is important to note that, due to the model
design during the first phase, the movement of
the wall adjacent to the rigid block caused a
small boundary effect that was reflected by the
slight distortion of the passive mark lines and eastward dragging of the south tip of the thrust faults in
the three models. This boundary effect, however,
did not affect the model results significantly.
Evolution of deformation in the second
phase
Figure 10 portrays the model evolution during
the second phase of deformation (left-lateral
transpression). In this case the bulk oblique
shortening (b.o.s.) is calculated as the percentage
of the moving wall displacement with respect to
the resulting length of the model in the same
direction (414 mm).
In the models Chortis 02 and Chortis 03, during
the first 5.8% b.o.s. (24 mm), the transpressional
deformation was accommodated by a major left
reverse-slip fault orthogonal to the first phase structures and parallel to the moving wall. Minor faults
and folds formed north and south of the main
thrust, with an angle of 458 to the trend of this
structure and a length of 2 cm (Fig. 10b & c).
Pre-existing thrust faults formed during the first
deformation phase started to rotate counterclockwise close to the moving wall during transpression;
a similar rotation pattern characterized the passive
markers on the model surface. Notably during the
second phase, deformation at the eastern side of
the block caused the development of a forelandverging thrust that progressively propagated to
the NW following the block boundaries.
In the case of model Chortis 04, the clay block
behaved as a rigid indenter in which the deformation was controlled by the high strength contrast with respect to the adjacent thinner brittle
crust. No structures formed within the block
and the deformation propagated at the block
boundaries with a higher velocity; at 3% b.o.s.,
all the structures showed in Figure 10j were
already formed. Around the block, thrusts
formed both at the clay –sand boundary and at
130
M. CERCA ET AL.
Fig. 9. Surface-view evolution of structures in the two phases presented in the form of a schematic table in Figures 4 and 5.
The three experiments are presented in columns and consecutive east-directed shortening are presented in lines. The
shaded area in line drawings corresponds to the modelled thicker crustal block. Symbols of structures as in Figure 4.
2 cm in front of the rigid block. At the western
side of the block, both these thrust sets showed a
vergence towards the hinterland, whereas the
thrusts at the eastern side were characterized by
a double vergence. At 5.8% b.o.s., the rigid
block had moved approximately 1 cm to the
NE, as indicated by distortion of the passive
grid (Fig. 10j).
COUPLING DURING LARAMIDE SHORTENING
Fig. 10. Surface-view evolution of structures during the second phase of oblique shortening.
131
132
M. CERCA ET AL.
Progressive transpression in models Chortis 02
and Chortis 03 was expressed by the continued
movement on the main left reverse-slip fault
and by further rotation of pre-existing structures
(Fig. 10c & g). This rotation also affected the
oblique faults and folds of the second phase of
deformation and progressively reduced their
angle to the main fault until they became
aligned with its trace. Slight distortion of the
passive markers and offset of the main fault
suggest a small counterclockwise rotation of the
rigid block. In model Chortis 04, the most important effect during this deformation interval was
that movement of the rigid block a further
1 cm to the NE caused left-lateral strike-slip
structures to form. Distortion of the passive grid
indicated a small clockwise rotation of the block
that induced the development of an extensional
basin at the west side of the block (Fig. 10k).
At the end of the experiments (17.4% b.s.,
72 mm of compression), the thrusts developed
in models Chortis 02 and 03 at the eastern
margin of the rigid block have propagated
toward the NE and reached the northern part of
the block; distortion of the passive markers indicates also right-lateral strike-slip faults along this
margin (Fig. 10d & h). In model Chortis 04, the
total displacement of the rigid block to the NE
was 3 cm and the clockwise rotation reached
98. A part of the block close to the moving
wall was slightly uplifted and the basin in the
west side of the block doubled its area. Counterclockwise rotation of the first phase thrusts was
also evident on this side of the block at the end
of the experiment.
Longitudinal sections
Ten longitudinal cross-sections representative of
the final stage of model deformation are shown in
Figure 11a, c and i. These sections highlight the
different influence of the two phases of deformation
in different parts of the model, defining three distinct regions: (1) a zone extending northward
beyond the rigid block, affected only by the first
phase of deformation (Fig. 11b, f & j); (2) a zone
corresponding to the northern part of the block
affected by structures belonging to both the two
phases of deformation (Fig. 11c, g & l); and (3) a
southern zone close to the contact between the
rigid block and the second phase moving wall
characterized by structures formed mainly during
the second phase of deformation (Fig. 11d, h & m).
In zone 1, shortening resulted in the development of thrusts and box-type folds; thrusts with
a prevailing vergence to the foreland, propagated
up to 9 cm in front of the moving wall (Fig. 11b,
f & j). Because of the high strength contrast
related to the presence of the rigid block, deformation propagated for only a short distance in
front of the moving wall in zones 2 and 3.
Zone 2, corresponding to the northern part of
the block, displays thrusts with opposing vergence. In the western side of the block, thrust
faults related to the fold-and-thrust belt are
characterized by vergence to the foreland; conversely, thrusts in the eastern side of the block
show a vergence toward the moving wall
(Fig. 11c, g & l). These latter structures are
formed during the second phase of deformation.
In zone 3, cross-sections display structures
formed mainly during the second phase of deformation (Fig. 11d, h & m). Longitudinal sections
of model Chortis 04 (Fig. 11k) show how the clay
block acted as a rigid indenter causing high-angle
reverse faults with vergence towards the block
and crust uplift at its boundaries. The block
remained undeformed as shown in all three
sections (Fig. 11d, h & m).
Summary of results
The general structural pattern that resulted from the
progressive deformation of the models is presented
schematically in Figure 12. During the first phase,
a fold-and-thrust belt with a dominant vergence
towards the foreland formed parallel to the
moving wall. Apart from model Chortis 04, the
second phase deformation was mainly accommodated by the formation of a left reverse-slip fault
orthogonal to the trend of the first phase faults,
and by lateral translation and rotation of the rigid
block. The higher cohesion of the clay block in
model Chortis 04 prevented internal deformation
and caused indentation in the adjacent crust. In
all the experiments, a second system of thrusts
nucleated at the eastern margin of the thicker
crustal block and then propagated northwestward
in domain I (Fig. 12). With increasing deformation,
a second fault system was observed in the west side
of the rigid block, domain II. Vertical-axis counterclockwise rotation of structures was observed in all
three models in the interference zone between the
two phases of deformation. Additionally, the clay
block in model Chortis 04 rotated clockwise
during progressive deformation; this rotation determined the development of an extensional basin and
NE-striking left-lateral faults in domain II.
Qualitative comparison of model results
with the geology
Limitations of modelling
Many natural parameters concerning the rheology and the boundary conditions of the
COUPLING DURING LARAMIDE SHORTENING
Fig. 11. Photographs of longitudinal sections.
133
134
M. CERCA ET AL.
Fig. 12. Simplified map of resulting domains of
deformation. Arrows indicate the direction of
convergence.
deformation process under investigation are not
easy to obtain and frequently data in the literature
are scarce, as is the case in southern Mexico. As a
consequence, analogue modelling necessarily
simplifies the geometry and the rheology of the
complex natural process and these simplifications have to be made explicit before comparing
the results with the geology.
Geometrical simplifications of the current
models involve the simulation of the Laramide
orogeny that caused deformation in a wide
fold-and-thrust belt in southern Mexico. This
deformation phase has been attributed to mechanical coupling between a subhorizontally subducted slab and an overriding continental crust
(Dickinson et al. 1988; Bird 1998). However, it
has been observed that analogue models shortened by advancing a rigid vertical boundary
laterally simulate most of the characteristics of
fold-and-thrust belts (Bonini 2001). In the
second phase, the deformation is attributed to
the motion of the Chortis block after its partial
detachment from North America in the Early
Tertiary (Herrmann et al. 1994). The shear
zone bounding the Xolapa complex to the north
has an ESE strike, but, as structures observed
in the field are mostly compatible with a transpressive regime, the movement of the wall was
made to simulate an ENE direction of the contraction, and variations in the plate boundary
through time were not considered. During the
experiments, the compressive stresses causing
model deformation are transmitted from the
rigid moving wall, thus simulating a lateral transmission of forces from the plate boundaries. This
represents a simplification of the natural process,
where forces causing deformation of the lithosphere at plate boundaries may be transmitted
vertically from below, driven by deformation of
the mantle (Tikoff & Teyssier 1998). Addition- Q2
ally, in all the models, the rigid block was
attached to the moving wall during transpression,
preventing important block rotations likely to
have occurred in the natural prototype.
From a rheological point of view, the simple
two-layer structure of the model, which is
intended to simulate the crust only, is a further
simplification of the natural situation. The experimental series was designed to investigate the
deformation around a strong (rigid) crustal
block (MOJB) embedded within a ‘normal’ continental crust. Since the crustal strength resides in
the upper brittle layer, our model set-up
considered a uniformly thick continental crust
characterized by a region with thicker brittle
crust. This implies that in the models the
ductile crust below the rigid block is thinner
than elsewhere. Unless the crust and mantle are
decoupled, the compression of two adjacent
crustal sections with different thicknesses may
result in the indentation of the upper mantle of
the thinner block into the less rigid lower crust
of the thicker block (Harry et al. 1995).
In addition, the cohesion of clay used in model
Chortis 04 is not properly scaled and clearly
exceeds the rigidity of the natural prototype, so
that the clay block behaved as an indenter,
emphasizing deformations at block boundaries.
For this reason the results can only be qualitavely
compared to the structures in the field. Several
factors can have an influence on the structures
formed by deformation, such as erosion and
deposition in basins, pre-existing structures,
effect of pore pressure in the growth and propagation of structures, thermal evolution or isostasy
effects, none of which were considered in the
modelling. Nevertheless, despite the abovementioned simplifications, comparisons of
model results with Nature were useful in understanding the structural evolution.
Comparison with Nature
Although a single model cannot explain all the
structural complexity observed in Nature, the
combined results of our experiments simulated
most of the styles of deformation and largescale structures observed around the rigid
crustal block of the MOJB. Thus, these results
suggest that the processes affecting both model
and natural prototype were similar. We emphasize that our models are pertinent in a time interval from 88 to 36 Ma. Line drawings of
COUPLING DURING LARAMIDE SHORTENING
models compared with a schematic structural
map of the south of Mexico are presented in
Figure 13.
The fold-and-thrust belt (Fig. 13, structure 1)
of southern Mexico has been associated with
the amalgamation of tectonic blocks during the
Mesozoic or the Early Tertiary, the time of the
Laramide episode. Recent microstructural data
in the eastern Guerrero terrane suggest that the
135
Laramide structures have a double vergence
and they have been interpreted in terms of a progressive ductile shear (Salinas-Prieto et al.
2000). During our experiments, double vergence
was modelled and was accentuated by the presence of the rigid block.
The first effect of the second phase of deformation is counterclockwise rotation of the
thrusts and folds in the southwestern part of
Fig. 13. Comparison of models with the natural prototype. The natural prototype structures are based mainly on
Campa & Coney (1983), Sedlock et al. (1993), Ortega-Gutiérrez et al. (1999), Elı́as-Herrera & Ortega-Gutiérrez (2002),
Ham-Wong (1981) and our own field data.
136
M. CERCA ET AL.
the model (Fig. 13, structure 2). Similar effects
have been documented in the southern part of
the Guerrero –Morelos platform (Cerca & Ferrari
2001). Other evidence of Tertiary counterclockwise vertical-axis rotations in this area has been
inferred from the palaeomagnetic data from
Cretaceous carbonate sequences (Molina-Garza
et al. 2003).
Vertical-axis rotations and lateral translation
of the high-strength block in the model mirror
complex structures and thrusting of the rigid
block over the adjacent crust at Papalutla
(Fig. 13, structure 3), consistent with the geometry of the Papalutla thrust and related deformation in the Early Tertiary basins to the NW
(Cerca & Ferrari 2001). Progressive strain at
the eastern margin of the rigid block produced
a thrust that propagated to the NW closely
following the geometry of the block. This structure strikingly resembles the geometry and the
kinematics of the Vista Hermosa fault (Fig. 13,
structure 4), which has thrust Palaeozoic schists
over Jurassic rocks (Sedlock et al. 1993).
To the north of the rigid block in the experiments, uplift and arcuate folding of the adjacent
part of the model were observed. This pattern of
deformation, clearly influenced by the geometry
of the rigid block, was emphasized in the model
with the clay block. Similar structures were
observed in the natural prototype in the Tentzo
Ranges (Fig. 13, structure 5) where folds of
Cretaceous carbonates define an arc convex
toward the north. Although most of these folds
were produced by the décollement of the carbonate succession, this process was likely triggered
by the uplift of the basement as simulated in the
model. As in the case of the Papalutla fault, the
Tentzo Ranges were considered to be produced
during the Laramide orogeny (Monroy & Sosa
1984). However, their anomalous orientation
with respect to the general trend of the Laramide
structures has not hitherto been explained.
One obvious difference between analogue
models and southern Mexico is that the structures
observed within the MOJB did not develop in the
models. These structures can be explained as
reactivation of discontinuities existing before
the Laramide event. One major example is the
Oaxaca fault, which exhibits a complex history
of lateral, inverse and normal movements beginning at least from the Jurassic or even the Palaeozoic time (Alaniz-Alvarez et al. 1994, 1996).
Only tight folds and strike-slip faults affecting
Tertiary red beds (Fig. 13, structure 6) resemble
structures formed in the models. Finally, in the
case of the model Chortis 04, left-lateral strikeslip and a basin are formed in the southwestern
side of the block (Fig. 13, structure 7) coincident
with the deposition of Tertiary red beds in the
eastern Guerrero state.
Vertical coupling and decoupling in the crust
of southern Mexico during the Late
Cretaceous and the Tertiary
Our new fieldwork linked with previous research
has recognized three deformation phases in
southern Mexico: Late Cretaceous (Laramide)
shortening, Early Tertiary left-lateral transpression,
and post-Eocene transtension. A series of analogue models have been used to simulate the
role of a rigid and thicker block (MOJB) within
the upper crust during the first two deformation
phases. The design of the models implies that
the resulting structural pattern is mainly
controlled by the strength of the upper brittle
crust and the forces applied at the vertical boundaries of the MOJB. Although these conditions
are given a priori, we believe that this could be
the real case. Indeed it is likely that during the
Laramide orogeny the mantle lithosphere was
removed or at least weakened by the subhorizontal subduction of the Farallon plate. If this
is the case, in the following stage the upper
crust remained the most rigid part of the whole
system. This view supports the claim of Jackson
(2002) that the detailed patterns of surface faulting in orogenic zones are predominantly controlled by the strength of the upper crustal blocks
and by the faults bounding them rather than by
forces applied at the base of the lithosphere.
In addition, in the models, decoupling between
ductile and brittle layers was enhanced by the
presence of the block. The higher strength and
thickness of the block increase the mechanical
contrast between brittle and ductile layers and
hence the decoupling between lower and upper
layers. This causes a propagation of strain in a
wide area around the block during the second
deformation phase. Thus, in our models, coupling or decoupling between the ductile and
brittle layers are determined by the thickness
and the rigidity of the upper crustal layer.
In Nature, southern Mexico was subject to a
general left-lateral strike slip regime along the
developing Caribbean –North America plate
boundary. During this Early Tertiary phase,
strain in the lower crust should have been distributed homogeneously in the deformation zone
between the two plates. By contrast, the deformation observed in the upper crust was focused
on a small number of structures around the
MOJB. This radically different pattern of deformation implies a small degree of coupling
between upper and lower crust.
COUPLING DURING LARAMIDE SHORTENING
Conclusions
We have performed a series of analogue modelling experiments to simulate the effect of two
deformation phases on a layered brittle–ductile
modelled crust incorporating a thicker upper
crustal block. The block was constructed with
either sand or clay to compare the mechanical
anisotropy effects of a high contrast in strength
and cohesion. Models reproduced most of the
structures observed around the MOJB in Nature,
suggesting a close similarity in deformation processes between Nature and model, and permit the
reinterpretation of several key features of the
Early Tertiary geological evolution of southern
Mexico. We identify structures consistent with
a transpressive regime during the second phase,
which interfere with the structures formed previously in the Laramide event. We show that
lateral partitioning of phase 2 strain is important
north of the Xolapa complex, in contrast with
previous interpretations that assign these structures to the Laramide deformation. The models
predict a Tertiary motion along the Vista
Hermosa fault, and that the structure propagated
towards the NW following the margin of the
MOJB. In the models, the propagation of deformation to the north is related to mechanical contrasts in strength and cohesion of the block with
respect to the adjacent modelled crust.
Structures formed in the tectonic interference
zone present counterclockwise vertical-axis
rotations consistent with the field evidence and
palaeomagnetic data. Structures within the MOJB
observed in Nature do not develop in the model.
This is likely due to reactivation of pre-existing
structures that influenced the deformation within
the block, a factor not considered in the model.
Without pre-existing structures the strength of
the block increases as a function of the thickness.
The experiments show that most of the structures
observed in Nature can be reproduced using relatively few parameters and a simple two-layer
analogue model. In our models, the degree of
coupling between the ductile and brittle layers
is controlled by the thickness and the rigidity of
the upper crustal layer. The Early Tertiary structural patterns observed in southern Mexico
around the MOJB suggest that it behaved similarly to the model.
This research was supported by grant CONACyT 32509-T
(to LF) and CONACYT– CNR bilateral grant. We thank
Giovana Moratti, Chiara Delventisette and Domenico
Montanari for their support and help during the laboratory
work. Guido Schreurs, John Grocott, Dora CarreónFreyre, Susana Alaniz and an anonymous reviewer provided constructive criticisms that enormously improved
137
this work. MC also thanks CONACYT, whose grants
permitted him to pursue his PhD, and a research stay in
Italy.
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