Download The North America±Caribbean plate boundary west of the Motagua

Journal of South American Earth Sciences 13 (2000) 459±468
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The North America±Caribbean plate boundary west of the
Motagua±Polochic fault system: a fault jog in Southeastern Mexico
M. GuzmaÂn-Speziale a,*, J.J. Meneses-Rocha b
a
b
Unidad de InvestigacioÂn en Ciencias de la Tierra, Instituto de GeofõÂsica, UNAM, Juriquilla, QuereÂtaro, Mexico
CoordinacioÂn de Estrategias de ExploracioÂn, PEMEX ExploracioÂn y ProduccioÂn,Villahermosa, Tabasco, Mexico
Abstract
We propose a model for the western end of the North American±Caribbean plate boundary. We suggest that, beyond the surface trace of
the Motagua±Polochic fault system, interplate strain is distributed along the Reverse Faults Tectonic Province, a zone of long, narrow
anticlines cut along their ¯anks by reverse faults that generally eliminate the intervening synclines, and the strike-slip faults of southeastern
Mexico, a system of at least nine major faults with left-lateral displacement and documented seismic activity. The reverse faults act as a
stepover (fault jog) between the strike-slip faults and the Motagua±Polochic system. Comparison with available stress data and models of the
stress ®eld at a stepover agree well with the observed pattern of folding and faulting in the area. Proposed displacement and seismicity along
each individual fault in the SE Mexico strike-slip system must be small because interplate strain is shared between at least seven major strikeslip faults. We suggest that motion between North America and the Caribbean dies out at the northwestern end of the strike-slip faults.
q 2000 Elsevier Science Ltd. All rights reserved.
Resumen
Se propone un modelo para la terminacioÂn occidental del lõÂmite de placas NorteameÂrica±Caribe. Se sugiere que, al oeste de la traza del
sistema de falllas Motagua±Polochic, la deformacioÂn interplaca esta distribuida a lo largo de la provincia tectoÂnica de Fallas Inversas, que es
una zona de anticlinales largos y angostos cortados en sus ¯ancos por fallas inversas que eliminan los sinclinales, y a lo largo de las Fallas de
Transcurrencia del sureste de MeÂxico, un sistema de al menos nueve fallas con corrimiento lateral izquierdo y actividad sõÂsmica documentada. La provincia de Fallas Inversas actuÂa como un escaloÂn entre las Fallas de Transcurrencia y el sistema Motagua±Polochic. El patroÂn de
plegamiento y fallamiento observado en el area concuerda con los modelos de estado de esfuerzos propuestos para escalones de este tipo. El
desplazamiento y la sismicidad a lo largo de cada una de las fallas en el sistema de Fallas Transcurrentes debe ser pequenÄo ya que la
deformacioÂn interplaca es compartida entre al menos siete de las mayores fallas del sistema. El modelo presentado tambieÂn propone que el
movimiento relativo entre NorteameÂrica y el Caribe se disipa en el extremo noroccidental de las Fallas de Transcurrencia. q 2000 Elsevier
Science Ltd. All rights reserved.
Keywords: Motagua±Polochic fault system; seismic activity; Strike-Slip Faults
1. Introduction
The plate boundary between the Caribbean and North
America plates consists, from east to west, of: the Puerto
Rico Trench, the Oriente fault zone, the Mid-Cayman
spreading system, the Swan fault zone, and the Motagua
and Polochic faults in Guatemala (Fig. 1). In westernmost
Guatemala, the surface trace of the Motagua Fault is lost at
the Tertiary volcanic deposits, and the Polochic Fault dies
out against the Chiapas Massif, a Permo-Triassic batholith
(Fig. 1).
* Corresponding author. Tel: 152-4-238-1119; fax: 152-4-238-1101.
E-mail address: [email protected] (M. GuzmaÂn-Speziale).
Much debate exists about the western continuation of the
plate boundary and the nature and location of the triple
junction of the North America, Cocos, and Caribbean plates
(see GuzmaÂn-Speziale et al., 1989). Most authors have
proposed that either the Motagua Fault (Malfait and Dinkelman, 1972; Plafker, 1976) or the Polochic Fault (Muehlberger and Ritchie, 1975; Burkart, 1978; SaÂnchez Barreda,
1981; Burkart, 1983; Machorro and Mickus, 1993) extends
west of its known surface trace and intersects the Middle
America Trench. GuzmaÂn-Speziale et al. (1989) argued that
there is no evidence for this intersection. The continuation
of either of these faults to the west poses serious problems:
the western end of the Motagua Fault behaved as a fault
terminus during the mainshock-aftershock sequence of the
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460
M. GuzmaÂn-Speziale, J.J. Meneses-Rocha / Journal of South American Earth Sciences 13 (2000) 459±468
Fig. 1. Tectonic setting of the study area. (a) The North America-Caribbean plate boundary region. Large arrows show direction of relative motion between
North American and Caribbean plates. Hatchured area is the Cayman Trough. (b) Main tectonic elements in southeastern Mexico. (c) Model of GuzmaÂnSpeziale et al. (1989) for the North America-Caribbean plate boundary.
great (Ms ˆ 7.5) Guatemalan earthquake of 1976 (see
Langer and Bollinger, 1979). Also, if the Polochic Fault
were to continue west of its known surface trace it would
have to cut through the Permo-Triassic Chiapas Massif, for
which there is no evidence. Rather, the plate boundary is
more likely to continue along the strike-slip faults of southeastern Mexico (GuzmaÂn-Speziale et al., 1989) (Fig. 1).
These faults are located northwest of the Motagua±
Polochic system and they have seismic activity with left-
lateral displacement, as does the Motagua±Polochic system.
A possible problem with this interpretation is that the strikeslip faults of southern Mexico are not directly aligned with
either the Motagua or the Polochic faults: there is an offset
of at least 100 km and their general orientation is from E±W
to NW±SE, rather than NE±SW to E±W as for the Motagua±Polochic system (Fig. 1).
In this paper, we propose a mechanism by which these
two fault systems may be connected. The feasibility of this
M. GuzmaÂn-Speziale, J.J. Meneses-Rocha / Journal of South American Earth Sciences 13 (2000) 459±468
461
Fig. 2. Generalizaed structural, lithological, and stratigraphic divisions of the Strike-Slip and Reverse-faults provinces. After Meneses-Rocha (1985).
scheme is tested by comparing the observed stress ®eld with
theoretical models.
2. Tectonic setting
The tectonics of the area is dominated by the interaction of three plates: North American, Cocos, and
Caribbean. The Cocos plate subducts the North American and Caribbean plates along the Middle American
Trench, whereas motion along the North American±
Caribbean plate boundary is resolved along the Motagua±Polochic fault system (Fig. 1). Other regional
elements also play a major role. Below, we brie¯y
describe the tectonic provinces of the region (Fig. 1).
2.1. The Strike-Slip Faults province
This tectonic province covers most of the Sierra de Chiapas (SaÂnchez-Montes de Oca, 1979). It is formed by a set of
rising and sinking blocks, bounded by left-lateral strike-slip
faults. NW-trending en echelon anticlines are present in
most of the uplifted blocks, with Mid- to Upper Cretaceous
and Paleogene rocks along their crests. Basins ®lled with
Cenozoic rocks formed where strike-slip faults diverge or
present double bends.
In the eastern part of the province, the faults trend east±west,
whereas in the central and western areas the faults are oriented
in a general N508W direction. In the westernmost part, the
faults die out in a horse tail (SaÂnchez-Montes de Oca, 1979;
Meneses-Rocha, 1985, 1991). The width of this shear belt is
about 100 km and its length is approximately 350 km.
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M. GuzmaÂn-Speziale, J.J. Meneses-Rocha / Journal of South American Earth Sciences 13 (2000) 459±468
There is evidence that the faults have left-lateral displacement, from focal mechanisms (GuzmaÂn-Speziale et al.,
1989), horizontal slickensides, en echelon patterns of
folds, and zones of compression (tension) where the faults
change in direction or form steps (Meneses-Rocha, 1985,
1991).
Stratigraphy suggests that the Strike-Slip Fault province
had developed throughout a complex history that involves
high-angle thrusting by Campanian time; a transition from a
phase of basin deepening to a phase of basin emergence
during Paleocene-Early Eocene time; and vertical motion
along fault-bounding blocks from the mid-Eocene to the
early Miocene (Meneses-Rocha, 1991). Motions on the
faults became de®nitely strike-slip during the middle
Miocene. From then to the present, there has been an alternation of transtensional and transpressional phases, giving
rise to the present structural pattern.
There is no general consensus regarding the amount of
left-lateral displacement. Viniegra (1971) mentioned displacements of the order of 10 km. SaÂnchez-Montes de Oca
(1979) estimates displacements of hundreds of kilometers
along each fault, using deformation of the intervening
synclines as evidence. However, based on mapping and
®eld work, Meneses-Rocha (1985, 1991) estimates the
offset across each fault to be 4±5 km in the eastern part
and 1±16 km across the central part. This gives an average
offset of 27 km for the eastern faults and 43 km for the
central faults, for a total displacement of about 70 km
since the late Miocene.
There are nine major strike-slip faults in the tectonic
province; the longest (TecpataÂn±Ocosingo, Malpaso, and
TelestaquõÂn±San Cristobal) are in the range 120±170 km
long.
2.2. Reverse Faults province
This tectonic province occupies the eastern portion of the
Sierra de Chiapas and the north-central part of Guatemala.
In Chiapas, it is equivalent to the Miramar and Yaxchilan
tectonic provinces of SaÂnchez-Montes de Oca (1979), while
in Guatemala it is equivalent to the Arco de la Libertad and
the Chapayal Basin of Vinson and Brineman (1963).
This province is a wide z-shaped structural trend formed
by long, narrow anticlines cut along their ¯anks by reverse
faults that generally eliminate the intervening synclines.
Fan- and box-shaped folds, or huge asymmetric anticlines
without a general trend of vergence are present in this
province. Normal faults divide the anticlines into upper
and lower blocks. Other normal faults slightly displace the
fold axes and traces of some reverse faults. Most of the
faults expose Upper Cretaceous carbonates along their
crests, and Tertiary terrigenous clastics cover the surface
of the synclines. The fact that middle Miocene terrigenous
clastics are involved in folding suggests a post-middle
Miocene age of folding and faulting. This is concomitant
with strike-slip displacement in the Strike-Slip Faults
Province.
Meneses-Rocha (1991) points out fundamental stratigraphic differences between this province and the StrikeSlip Faults province. The Lower to mid-Cretaceous strata
in the Reverse Faults Province mostly contain incompetent
rocks (anhydrites and minor dolomites), whereas in the
Strike-Slip Faults Province this sequence is made up of
competent rocks in its western and central areas, with
incompetent rocks only in its eastern portion, adjacent to
the Reverse Faults (Fig. 2). Thus, in the Reverse Faults
Province two levels of incompetent beds produce two potential detachment horizons, at the level of the Middle Jurassic
salt deposits and along the Lower to mid-Cretaceous
evaporites. In most of the Strike-Slip Faults Province, the
Middle-Jurassic salt deposits is the only possible zone of
detachment.
2.3. Motagua and Polochic faults
The Motagua and Polochic faults cross central Guatemala
in an arc concave to the north (Fig. 1). Seismic activity and
geomorphic features demonstrate left-hand slip across the
Polochic fault during the Quaternary (e.g. Burkart, 1978;
Schwartz et al., 1979; Burkart, 1983; GuzmaÂn-Speziale et
al., 1989; White, 1985). However, there is no agreement on
the time and amount of its main displacement. Erdlac and
Anderson (1982) and Anderson et al. (1985) have interpreted the Polochic Fault as an important pre-Turonian
strike-slip fault, with only a few kilometers of post-Cretaceous slip; but Burkart (1978, 1983) and Deaton and Burkart
(1984) have argued that sinistral slip across the Polochic
Fault began after the Mid-Miocene and concluded before
the Pliocene, resulting in a displacement of 130 km.
Several plate tectonic reconstructions (e.g. Malfait and
Dinkelman, 1972; Pindell and Dewey, 1982; Wadge and
Burke, 1983) imply that large sinistral slip along the
Polochic and Motagua faults also occurred during the Paleogene, as spreading along the Cayman Trough started,
although there is no ®eld evidence for this displacement.
Recent seismic activity along the Motagua (e.g. Plafker,
1976; Kanamori and Stewart, 1978) and Polochic (White,
1985) faults indicate that both are active at present.
2.4. The Chiapas Batholith
The Chiapas Batholith or Chiapas Massif extends for
about 300 km in a NW±SE direction, roughly parallel to
the Middle America Trench, from the Isthmus of Tehuantepec to the Guatemala border (Fig. 1). Its mean altitude is
2000 m with a width of about 75 km. The main intrusion is
composed of granites and granodiorites of Paleozoic (probably Carboniferous or Permian) age, although locally there
are metamorphic rocks which range in age from Precambrian to Miocene (Dengo, 1968; LoÂpez-Ramos, 1981).
M. GuzmaÂn-Speziale, J.J. Meneses-Rocha / Journal of South American Earth Sciences 13 (2000) 459±468
463
Table 1
Speed and azimuth of relative plate motion of the North American plate with respect to the Caribbean plate at a point P1 (16.08, 291.58), and at point P2 (17.258,
294.08) (N ˆ Pole Number. References: 1 ˆ Jordan (1975), 2 ˆ RM2 from Minster and Jordan (1978), 3 ˆ Sykes et al. (1982), 4 ˆ Stein et al. (1988),
5 ˆ NUVEL-1 from DeMets et al. (1990), 6 ˆ Deng and Sykes (1995), 7 ˆ Dixon et al. (1998))
N
Pole
Lat. 8N
Lat. 8E
v 8/my
P1
Speed cm/yr
Azimuth
P2
Speed cm/yr
Azimuth
1
2
3
4
5
6
7a
7b
50.0
2 33.8
66.0
2 55.2
2 74.3
68.4
18.6
26.4
116.0
2 70.5
2 132.0
2 60.8
2 26.1
2 126.3
107.2
109.7
0.20
0.22
0.36
0.11
0.11
0.25
0.36
0.27
2.1
2.0
3.3
1.2
1.2
2.3
2.5
2.2
251.6
244.7
251.6
240.0
255.6
255.5
241.3
243.8
2.1
2.0
3.3
1.2
1.2
2.3
2.7
2.3
250.3
246.4
252.2
251.4
255.2
256.1
238.8
241.7
3. Relative plate motion
Motion between the Caribbean and North American
plates features subduction along the Puerto Rican Trench,
sea-¯oor spreading on the Mid-Cayman Spreading Center,
and transform displacement along the Oriente, Swan and
Motagua and Polochic fault zones (Fig. 1). Different authors
have used different data sets trying to determine the pole of
rotation between the plates. MacDonald (1976) ®tted small
circles to the Oriente and Swan Island faults; Sykes et al.
(1982) and Deng and Sykes (1995) used earthquake slip
vectors from the Puerto Rico Trench and the transform
faults. Minster and Jordan (1978) and Stein et al. (1988)
included spreading rates, slip vectors, and fault orientations
in their models. Finally, Calais and De LeÂpinay (1993)
applied a deformation pattern scheme to determine the
pole of rotation. None of the pole ®t all of the features
that make up the plate boundary (see Heubeck and Mann,
1991; Calais and De LeÂpinay, 1993 for a review). Poles
determined by different authors are widely scattered from
northern North America to eastern Asia. Deng and Sykes
(1995) noted that all poles lie close to a great circle perpendicular to the Cayman Trough. DeMets (1993) noted that
motion between the Caribbean and neighboring plates
cannot be ®tted by reasonable Euler vectors, no matter
which data set is used.
While different poles yield different speeds, they match
the azimuth of relative motion. Using several published
Euler poles, we calculated the speed and azimuth of relative
plate motion between the Caribbean and North American
plates at a point located at (16.08, 291.58), which is near the
terminus of the Motagua±Polochic fault system, and at a
point located at (17.258, 294.08), near the western end of the
Strike-Slip Faults Province (Table 1). Speeds range from
12±33 mm/yr, and the azimuths are consistently similar
(t2508).
According to Heubeck and Mann (1991), the absence of a
consistent Euler vector between the North American and
Caribbean Plates suggests that the northern Caribbean
Plate boundary may be modeled by the movement of three
distinct rigid blocks within it, but recent GPS work suggests
that the Caribbean Plate behaves as a single rigid plate (T.
Dixon, written communication, 1998).
4. Seismicity and state of stress
4.1. Seismicity
Practically along all of its length, the North American±
Caribbean Plate boundary is seismically active (e.g.
McCann and Pennington, 1990; Dewey and SuaÂrez, 1991).
The Motagua±Polochic fault system is particularly active
(Fig. 3), as evidenced by the great 1976 earthquake
(Ms ˆ 7.5) that ruptured a 230 km-long segment of the
Motagua Fault (Plafker, 1976; Kanamori and Stewart,
1978). This fault ruptured in previous large earthquakes in
this century, such as the 1945 (Ms ˆ 5.7) and 1980
(Ms ˆ 6.4) events (White and Harlow, 1993). A great earthquake also occurred along the Polochic Fault in 1816. White
(1985) postulates that only one of the faults takes up the
relative motion at a given time.
The Strike-Slip Faults Province is also seismically active.
Catalogs of modern seismicity from 1964 show frequent
moderate-size, shallow events. Most of the available faultplane solutions (e.g. GuzmaÂn-Speziale et al., 1989) are leftlateral strike-slip mechanisms, which are parallel to the
strike of the faults. Figueroa (1970), Ganse and Nelson
(1981), Abe and Noguchi (1983), Pacheco and Sykes
(1992), and GarcõÂa Acosta and SuaÂrez (1996) cite large
historical earthquakes, that were probably located along
the Strike-Slip Faults. The epicenters of events in 1902,
1914, 1927, 1935, and 1937 were located directly on the
faults (Fig. 3), although some may be related to the
subducted Cocos Plate. Figueroa (1973) reports that the
1902 event `is the largest recorded in Chiapas'. It nearly
destroyed the town of San Bartolome de los Llanos (now
Venustiano Carranza). The 1914 event caused severe
damages in the towns of Altamirano, Ocosingo, and HuixtaÂn
(Figueroa, 1973), all on the Strike-Slip Faults, suggesting
that this event took place along one of the faults.
Seismicity in the Reverse Faults Province is lower than in
464
M. GuzmaÂn-Speziale, J.J. Meneses-Rocha / Journal of South American Earth Sciences 13 (2000) 459±468
Fig. 3. Seismicity in southeastern Mexico. Earthquake epicenters for events with depths less than or equal to 50 km and mb $ 4:5: (a) As reported by the
Servicio SismoloÂgico Nacional (SSN) (open circles). Also shown (solid circles) are historic events (without depth or magnitude constraints) from the catalogs
of: SSN (C. JimeÂnez, pers. comm., 1997), Figueroa (1970); Figueroa (1973); Ganse and Nelson (1981), and Pacheco and Sykes (1992). (b) As reported by the
ISC (1964±1986) and the NEIC (1973±1997).
the neighboring Strike-Slip Faults Province. The Servicio
SismoloÂgico Nacional (National Seismological Survey of
Mexico) only reports two events with mb $ 4:5 from 1974
to 1996, whereas National Earthquake Information
Center/International Seismological Centre (NEIC/ISC)
report less than a dozen in 1964 to 1996 (Fig. 3).
Historically, the only event probably related to the
Reverse Faults Province was the earthquake of 5 February, 1954, which nearly destroyed the town of YajaloÂn.
This event was probably shallow: it was located far
inland from the deepest earthquakes related to subduction of the Cocos Plate (Bevis and Isacks, 1984;
Burbach et al., 1984) and, according to newspaper
accounts of the time, damage was localized. GuzmaÂnSpeziale et al. (1989) did not ®nd focal mechanisms
associated with the Reverse Faults. Centroid-Moment
Tensor data (Dziewonski and Woodhouse, 1983) does
not include any mechanism for this tectonic province.
M. GuzmaÂn-Speziale, J.J. Meneses-Rocha / Journal of South American Earth Sciences 13 (2000) 459±468
Table 2
Parameters of fault-plane solutions in southeastern Mexico
Na
Date
Lat. 8N
Lon. 8E
Strike
Dip
Rake
Ref. b
1
2
3
4
5
6
7
8
9
19850211
19751010
19680805
19660129
19711012
19740826
19951219
19760204
19760209
17.07
17.18
17.17
16.36
15.74
15.97
15.27
15.27
15.39
2 94.74
2 93.90
2 92.47
2 91.35
2 91.24
2 91.04
2 90.06
2 89.25
2 89.11
210
290
12
158
108
288
267
156
274
66
72
60
84
80
60
73
85
68
2 22
2 11
2 165
2 10
18
33
24
180
11
1
2
2
2
3
2
1
4
2
a
N, locations shown in Fig. 4.
References: 1 ˆ Centroid Moment Tensor Solution from the Harvard
Seismology Group (Dziewonski and Woodhouse, 1983), 2 ˆ GuzmaÂnSpeziale et al., 1989, 3 ˆ Yamamoto, 1978, 4 ˆ Kanamori and Stewart,
1978.
b
4.2. State of stress
Generally, four types of indicators are used to determine
the tectonic stress ®eld (e.g. Zoback and Zoback 1991):
well-constrained earthquake focal mechanisms, drillhole
elongations, volcanic alignments, and fault-slip analysis.
For southeastern Mexico, fault-plane solutions (Kanamori
and Stewart, 1978; Yamamoto, 1978; Dziewonski and
Woodhouse, 1983; GuzmaÂn-Speziale et al., 1989), and drillhole elongations (Suter, 1991) are available.
Nine available focal mechanisms for the area (Table 2,
Fig. 4) show a left-lateral strike-slip mechanism. P-axes are
horizontal and oriented NE±SW to E±W, whereas T-axes,
also horizontal, are oriented NW±SE to N±S. Drillhole
elongation data (Suter, 1991) from locations just north of
the area of interest, show a general NNE±SSW trend for the
maximum horizontal stress (SH) (Fig. 4). This direction is
perpendicular to the general trend of folding and faulting in
the Reverse Faults Province.
465
studied examples occur along the San Andreas Fault System
in California where both positive and negative jogs are
present (e.g. Sharp and Clark, 1972; Johnson and Hadley,
1976; Sibson, 1986), or in Turkey, where the North and East
Anatolian faults meet (Barka and Kadinsky-Cade, 1988).
GuzmaÂn-Speziale et al. (1989) suggest that the StrikeSlip Faults Province of southeastern Mexico is part of the
North American±Caribbean Plate boundary. We propose
that the Motagua±Polochic system, the Strike-Slip Faults
of southeastern Mexico, and the Reverse Faults Province
form a fault jog. The sense of motion for the Motagua±
Polochic System and the faults of southeastern Mexico is
left-lateral, while the stepover is right-step. Therefore, a
compressional jog is expected. The intervening Reverse
Faults Tectonic Province displays an appropriate compressional stress distribution for being part of the jog.
In a compressional jog, the high compressive stress in the
inter-segment region prevents slip transfer across the step
and favors the deformation to spread beyond the intersegment region, with development of extensive folding
and faulting as well as localized updoming in the vicinity
of the jog (Segall and Pollard, 1980; Sibson, 1986; Scholz,
1990). Therefore, the Reverse Faults Province encompasses
an area beyond the Motagua±Polochic and Strike-Slip
Faults of southeastern Mexico. Folding and faulting only
developed on the northeastern side of the jog, in the Reverse
Faults Province. This may be due to the lack of sediments
and the presence of the competent Chiapas Massif on the
southwestern side, as opposed to the two-sided model of
Sibson (1986) (Fig. 4).
The characteristic that distinguishes this proposed jog
from others in the literature is that it does not occur on
two segments of a single fault. Rather, it connects two
major faults (Moatgua and Polochic) with nine medium
sized faults (the Strike-Slip Faults of southeastern Mexico).
6. Discussion
5. Proposed model
Fault jogs are perpendicular deviations of the fault from
its slip plane. They may occur on strike-slip, thrust, or
normal faults (Scholz, 1990). For a strike-slip fault, a jog
will be compressional (also called negative, antidilational,
or restraining) if the sense of the stepover is opposite the
sense of the fault. Otherwise, the jog is extensional (positive, dilational, or releasing). Thus, if the fault is left-lateral
and the stepover is right lateral the jog is compressional (e.g.
Segall and Pollard, 1980; Barka and Kadinsky-Cade, 1988;
Scholz, 1990). Main by-products of extensional jogs are
sedimentary basins (mostly pull-apart basins) while blocks
elevated by crustal shortening will be caused by compressional jogs (Segall and Pollard, 1980; Sibson, 1986; Scholz,
1990). These features have long been recognized along
different faults and on different scales. Perhaps the best
If the Motagua±Polochic fault system is the western end
of the North America±Caribbean plate boundary, where
does it continue? Current models propose that either the
Polochic or the Motagua faults intersect the Middle America
Trench west of their known surface traces. The problem is
that a trench±trench±transform triple junction is unstable
(McKenzie and Morgan, 1969). The trench is being rapidly
displaced by the transform fault forming two triple junctions
of the trench±transform±transform type. There is no
evidence that this had ever occurred along the Middle
American Trench. The trench has remained stable at least
since the Pliocene (e.g. Mammerickx and Klitgord, 1982;
Pindell and Dewey, 1982) and there is no geologic
evidence that suggests that either the Polochic or the
Motagua Fault continue to the west of their known surface
traces.
A fault jog may not be the simplest scenario for the
466
M. GuzmaÂn-Speziale, J.J. Meneses-Rocha / Journal of South American Earth Sciences 13 (2000) 459±468
Fig. 4. State of stress in southeastern Mexico. (a) Observed fault-plane solutions (numbers refer to Table 2), and directions of maximum horizontal stress (bars)
from borehole elongations, as reported by Suter (1991). (b) Theoretical structures within a compressional jog, as proposed by Sibson (1986).
westward continuation of the Motagua±Polochic fault
system, but it provides a better dynamic solution. Sediments of the Reverse Faults Province lie on top of two
salt horizons, of Callovian and Upper Jurassic to midCretaceous age. Davis and Engelder (1985) pointed out
that evaporites ªcan provide an extremely weak horizon
within which a basal detachment can form ¼º (p. 67).
The frictional strength of evaporites is much weaker
than of any other rock type (e.g. Davis and Engelder,
1985), thus providing less resistance to slip for the
formation of a fold-and-thrust belt. On the other hand,
an extension of the Motagua±Polochic fault system
(particularly the Polochic Fault) beyond the present
western end would have to cut the granite of the Chiapas Massif.
There is no single reason for the formation of fault jogs
(e.g. Segall and Pollard, 1980; Sibson, 1986; Scholz, 1990).
In southeastern Mexico, a fault jog may have developed
because the Motagua±Polochic fault system was unable to
propagate across the Chiapas Massif and favored the relatively shallow detachment horizon provided by the evaporites instead. Strain is transformed into strike-slip faulting as
the shallow detachment level grades into more competent
rocks.
This explanation solves the problem of the Motagua±
Polochic fault system, yet it translates the same problem
M. GuzmaÂn-Speziale, J.J. Meneses-Rocha / Journal of South American Earth Sciences 13 (2000) 459±468
to the western end of the Strike-Slip Faults. The faults, in
their western section, are oriented approximately N508W.
According to published Euler poles, the azimuth of motion
between the two plates near the western end of the faults
(17.258, 294.08) is ,S708W (see Table 1), which differs by
608 from N508W. In other words, the component of motion
along the faults is only 50% of the total motion. Furthermore, strain should be distributed along at least six faults;
thus, less than ten percent of the motion corresponds to any
one fault. Relative motion between the North America and
Caribbean Plates thus dies out in a horse tail.
The Strike-Slip Faults of southeastern Mexico must be a
part of the plate boundary, as originally proposed in
GuzmaÂn-Speziale et al. (1989). In this paper we propose
the Reverse-Faults tectonic province as a link between the
Strike-Slip Faults of southeastern Mexico and the Motagua±Polochic fault system.
7. Conclusions
A fault jog between the Motagua±Polochic fault system
and the Strike-Slip Faults Province of southeastern Mexico
is proposed. The sense of displacement is left-lateral and the
sense of the jog is right-lateral, in agreement with the
observed stress pattern. Low seismicity in the folded belt
indicates a plastic behavior.
We suggest that the jog was formed because incompetent
salt and anhydrite horizons in the Reverse Faults Province
provide a favorable detachment horizon, whereas the Chiapas Massif blocks the extension of the Polochic Fault.
Motion between the North American and Caribbean
Plates dies out at the western end of the Strike-Slip Faults
Tectonic Province, because of the orientation and large
number of faults in this region.
Acknowledgements
We are grateful to Wayne Pennington who meticulously
read the manuscript, signi®cantly improving it. We also
thank Tim Dixon for pointing out his latest work in the
area and providing valuable information. Vladimir Kostoglodov also read the manuscript and made additional
comments. This project was funded by the Consejo Nacional de Ciencia y TecnologõÂa (Conacyt-Mexico) through
grant 1757-T9210 and by Universidad Nacional AutoÂnoma
de MeÂxico (UNAM) through grant DGAPA-PAPIIT
IN107195, both to GuzmaÂn-Speziale. Unidad de InvestigacioÂn en Ciencias de la Tierra (UNICIT) contribution 77.
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