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Journal of South American Earth Sciences 13 (2000) 459±468 www.elsevier.nl/locate/jsames 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 0895-9811/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S 0895-981 1(00)00036-5 460 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 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. 462 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 Pole Number. References: 1 Jordan (1975), 2 RM2 from Minster and Jordan (1978), 3 Sykes et al. (1982), 4 Stein et al. (1988), 5 NUVEL-1 from DeMets et al. (1990), 6 Deng and Sykes (1995), 7 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 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 5.7) and 1980 (Ms 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 Centroid Moment Tensor Solution from the Harvard Seismology Group (Dziewonski and Woodhouse, 1983), 2 GuzmaÂnSpeziale et al., 1989, 3 Yamamoto, 1978, 4 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. References Abe, K., Noguchi, S., 1983. Revision of magnitudes of large shallow earthquakes, 1897±1912. Phys. Earth Planet. Interiors 33, 1±11. Anderson, T.H., Erdlac, R.J., Sandstorm, M.A., 1985. Late Cretaceous and 467 post Cretaceous strike-slip displacement along the Cuilco±Chixoy± Polochic fault, Guatemala. Tectonics 4, 453±475. Barka, A.A., Kadinsky-Cade, K., 1988. Strike-slip fault geometry in Turkey and its in¯uence on Earthquake activity. Tectonics 7, 663±684. Bevis, M., Isacks, B.L., 1984. Hypocentral trend surface analysis: Probing the geometry of Benioff zones. J. Geophys. Res. 89, 6153±6170. Burbach, G.V., Frohlich, C., Pennington, W.D., Matumoto, T., 1984. Seismicity and tectonics of the subducted Cocos Plate. J. Geophys. Res. 89, 7719±7735. Burkart, B., 1978. Offset across the Polochic fault of Guatemala and Chiapas, Mexico. Geology 6, 328±332. Burkart, B., 1983. Neogene North American-Caribbean plate boundary across northern Central America: offset along the Polochic fault. Tectonophyiscs 99, 251±270. Calais, E., De LeÂpinay, B.M., 1993. Semiquantitative modeling of strain and kinematics along the Caribbean/North America plate boundary zone. J. Geophys. Res. 98, 8293±8308. Davis, D.M., Engelder, T., 1985. The role of salt in fold-and-thrust belts. Tectonophysics 119, 67±88. Deaton, B.C., Burkart, B., 1984. Time of sinistral slip along the Polochic fault of Guatemala. Tectonophysics 102, 297±313. DeMets, C., 1993. Earthquake slip vectors and estimates of present-day plate motions. J. Geophys. Res. 98, 6703±6714. DeMets, C., Gordon, R.G., Argus, D.F., Stein, S., 1990. Current plate motions. Geophys. J. Int. 101, 425±478. Deng, J., Sykes, L.R., 1995. Determination of Euler pole for contemporary relative motion of Caribbean and North American plates using slip vectors of interplate earthquakes. Tectonics 14, 39±53. Dengo, G. (1968). Estructura geoloÂgica, historia tectoÂnica y morfologõÂa de AmeÂrica Central, 50 pp.. Centro Regional de Ayuda Teecnica. Agencia para el Desarrollo Internacional (AID). Mexico City. Dewey, J.W., SuaÂrez, G., 1991. Seismotectonics of Middle America. In: Slemmons, D.B., Engdahl, E.R., Zoback, M.D., Blackwell, D.D. (Eds.). Neotectonics of North America. Geological Society of America, Decade Map Volume 1. Boulder, Colorado, pp. 309±321. Dixon, T.H., Farina, F., DeMets, C., Jansma, P., Mannand, P., Calais, E., 1998. Relative motion between the Caribbean and North American plates and related boundary zone deformation from a decade of GPS observations. J. Geophys. Res. 103, 15157±15182. Dziewonski, A.M., Woodhouse, J.H., 1983. An experiment in systematic study of globaal seismicity: centroid-moment tensor solutions for 201 moderate and large earthquakes of 1981. J. Geophys. Res. 88, 3247± 3271. Erdlac Jr, R.J., Anderson, T.H., 1982. The Chixoy±Polochic fault and its associated fractures in western Guatemala. Geol. Soc. Am. Bull. 93, 57±67. Figueroa, J., 1970. CataÂlogo de sismos ocurridos en la repuÂblica mexicana. Instituto de IngenierõÂa, UNAM, Informe 272. MeÂxico, 88pp. Figueroa, J., 1973. Sismicidad en Chiapas. Instituo de IngenierõÂa, UNAM, Informe 316. MeÂxico, 52pp. Ganse, R.A., Nelson, J.B., 1981. Catalog of signi®cant earthquakes 2000 BC±1979 including quantitative casualties and damage. World Data Center A for Solid Earth Geophysics Report SE-27. Boulder, CO, 154pp. GarcõÂa Acosta, V., SuaÂrez, G., 1996. Los sismos en la historia de MeÂxico, I, Universidad Nacional AutoÂnoma de MeÂxico, Centro de Investigaciones y Estudios Superiores en AntropologõÂa Social, Fondo de Cultura EconoÂmica, MeÂxico, 718pp. GuzmaÂn-Speziale, M., Pennington, W.D., Matumoto, T., 1989. The triple junction of the North America Cocos, and Caribbean plates: Seismicity and tectonics. Tectonics 8, 981±997. Heubeck, C., Mann, P., 1991. Geologic evaluation of plate kinematic models for the North America±Caribbean plate boundary zone. Tectonophysics 191, 1±26. Johnson, C.E., Hadley, D.M., 1976. Tectonic implications of the Brawley earthquake swarm Imperial Valley, California, January 1975. Bulletin of the Seismological Society of America 66, 1132±1144. 468 M. GuzmaÂn-Speziale, J.J. Meneses-Rocha / Journal of South American Earth Sciences 13 (2000) 459±468 Jordan, T.H., 1975. The present-day motions of the Caribbean plate. J. Geophys. Res. 80, 4433±4439. Kanamori, H., Stewart, G., 1978. Seismological aspects of the Guatemala earthquake of February 4, 1976. J. Geophys. Res. 83, 3427±3434. Langer, C.J., Bollinger, G.A., 1979. Secondary faulting near the terminus of a seismogenic strike-slip fault: Aftershocks of the 1976 Guatemala earthquake. Bulletin of the Seismological Society of America 69, 427±444. LoÂpez Ramos, E., 1981. GeologõÂa de MeÂxico. EdicioÂn Escolar, MeÂxico, 446pp. MacDonald, W.D., 1976. Cretaceous-Tertiary evolution of the Caribbean. Transactions of the Caribbean Geological Conference 7, 69±78. Machorro, R., Mickus, K., 1993. Structural continuity of the Polochic Fault into southwest Mexico. Eos, Transactions of the American Geophysical Union 74, 576. Malfait, B.T., Dinkelman, M.G., 1972. Circum-Caribbean tectonic and igneous activity and the evolution of the Caribbean Plate. Geol. Soc. Am. Bull. 83, 251±272. Mammerickx, J., Klitgord, K.D., 1982. Northern East Paci®c Rise: Evolution from 25 m.y B. P. to the present. J. Geophys. Res. 87, 6751±6759. McCann, W.R., Pennington, W.D., 1990. Seismicity, large earthquakes, and the margin of the Caribbean Plate. In: Dengo, G., Case, J.E. (Eds.) The Caribbean region. The Geology of North America, Volume H. Boulder, CO. McKenzie, D.P., Morgan, W.J., 1969. Evolution of triple junctions. Nature 224, 125±133. Meneses-Rocha, J.J., 1985. Tectonic evolution of the strike-slip fault province of Chiapas, Mexico. M. A. thesis, University of Texas. Austin, TX. Meneses-Rocha, J.J., 1991. Tectonic development of the Ixtapa Graben, Chiapas, Mexico. PhD dissertation, University of Texas. Austin, TX. Minster, J.B., Jordan, T.H., 1978. Present-day plate motions. J. Geophys. Res. 83, 5331±5354. Muehlberger, W., Ritchie, A.W., 1975. Caribbean-Americas plate boundary in Guatemala and southern Mexico as seen on Skylab IV orbital photography. Geology 3, 232±235. Pacheco, J., Sykes, L., 1992. Seismic moment catalog of large shallow earthquakes 1900±1989. Bull. Seismol. Soc. Am. 82, 1306±1349. Pindell, J., Dewey, J.F., 1982. Permo-Triassic reconstruction of western Pangea and the evolution of the Gukf of Mexico/Caribbean region. Tectonics 1, 179±211. Plafker, G., 1976. Tectonic aspects of the Guatemala earthquake of 4 February 1976. Science 193, 1201±1208. SaÂnchez-Barreda, L.A., 1981. Geologic evolution of the continental margin of the gulf of Tehuantepec in southern Mexico. PhD dissertation, University of Texas. Austin, TX. SaÂnchez-Montes de Oca, R., 1979. Geologia petrolera de la sierra de Chiapas. BoletõÂn de la AsociacioÂn Mexicana de GeoÂlogos Petroleros, 31, 67±97. Schwartz, D.P., Cluff, L.S., Donnelly, T.W., 1979. Quaternary faulting along the Caribbean-North American plate boundary in Central America. Tectonophysics 52, 431±445. Scholz, C.H., 1990. The mechanics of earthquakes and faulting. Cambridge University Press, Cambridge, p. 461. Segall, P., Pollard, D.D., 1980. Mechanics of discontinuous faults. J. Geophys. Res. 85, 4337±4350. Sharp, R.V., Clark, M.M., 1972. Geologic evidence of previous faulting near the 1968 rupture on the Coyote Creek fault. US Geol. Surv. Professional Paper 787, 131±140. Sibson, R.H., 1986. Rupture interaction with fault jogs. In: Das, S., Boatwright, J., Scholz, C.H. (Eds.). Geophys. Monogr.. Earthquake source mechanics, 37. American Geophysical Union, Washington, DC, pp. 157±168. Stein, S., DeMets, C., Gordon, R.G., Brodholt, J., Argus, D., Engeln, J.F., Lundgren, P., Stein, C., Wiens, D.A., Woods, D.F., 1988. A test of alternative Caribbean Plate relative motion models. J. Geophys. Res. 93, 3041±3050. Suter, M., 1991. State of stress and active deformation in Mexico and western Central America. In: Slemmons, D.B., Engdahl, E.R., Zoback, M.D., Blackwell, D.D. (Eds.). Neotectonics of North America. Geological Society of America, Boulder, CO, pp. 401±421 (Decade Map Volume 1). Sykes, L.R., McCann, W.R., Kafka, A.L., 1982. Motion of Caribbean plate during last 7 million years and implications for earlier cenozoic movements. J. Geophys. Res. 87, 10656±10676. Viniegra, F., 1971. Age and evolution of salt basins of southeastern Mexico. Am. Assoc. Petroleum Geol. Bull. 55, 478±494. Vinson, G.L., Brineman, J.H., 1963. Nuclear Central America, hub of antillean transverse belt. Am. Assoc. Pet. Geol. Mem. 2, 101±113. Wadge, G., Burke, K., 1983. Neogene Caribbean plate rotation and associated Central American tectonic evolution. Tectonics 2, 633±643. White, R.A., 1985. The Guatemala earthquake of 1816 on the ChixoyPolochic Fault. Bull. Seismol. Soc. Am. 75, 455±473. White, R.A., Harlow, D.H., 1993. Destructive upper-crustal earthquakes of Central America since 1900. Bull. Seismol. Soc. Am. 83, 1115±1142. Yamamoto, J., 1978. Rupture processes of some complex earthquakes in southern Mexico, PhD dissertation, St. Louis University. St. Louis, MO. Zoback, M.D., Zoback, M.L., 1991. Tectonic stress ®eld of North America and relative plate motions. In: Slemmons, D.B., Engdahl, E.R., Zoback, M.D., Blackwell, D.D. (Eds.). Neotectonics of North America. Geological Society of America, Boulder, CO, pp. 339±366 (Decade Map Volume 1).