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Concordant Paleomagnetic Directions from the Tehuantepec Isthmus: Constraints on the Neogene Evolution of the North American-Caribbean Plate Boundary

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Concordant Paleomagnetic Directions from the Tehuantepec Isthmus: Constraints on the Neogene Evolution of the North American-Caribbean Plate Boundary TECTONOPHYSICS ELSEVIER Tectonophysics 235 (1994) 339-346 Concordant paleomagnetic directions from the Tehuantepec Isthmus: Constraints on the Neogene evolution of the North American-Caribbean plate boundary Roberto S. Molina-Carza ‘, Jairne Ur~tia-Fucugauchi b, Rob Van der Voo ’ a Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131-1116, USA b Laboratorio de Paleomagnetismo, Institute de Geofuica, UNAM, Coyoacan 04510 D.F., Mexico ‘Department of Geological Sciences, UniversiQ of Michigan, Ann Arbor, MI 4810%1063, USA Received 12 August, 1993; accepted 17 March 1994 Abstract We report paleomagnetic data for Miocene igneous rocks of the southeastern Tehuantepec Isthmus region, Mexico. Thermal and alternating field demagnetization reveal predominantly norma polarity magnetization, interpreted as primary TRM. The overall mean direction (Dee = 355.6”, Inc = 3&Y, k = 36.4, ag5 = 9.3”, N = 8 sites) is statistically indistinguishable from the expected direction, calculated using the North American reference pole. This result indicates that significant rotation or displacement of the Tehuantepec region relative to the craton has not occurred since 13 Ma. 1. Introduction 1979). Offshore, in the Caribbean sea, the bound- ary extends along the Cayman Trough (Holcombe Tertiary tectonics of southern Mexico and and Sharman, 1983). northern Central America has been related to the The region has presented many long standing interactions of the Cocos (Farallon), Caribbean, problems in explaining its tectonic evolution and and North American plates (Molnar and Sykes, even its present tectonic relationships. Several 1969; Jordan, 1975; Burkart, 1983). The Cocos lines of evidence on land point to a modest plate is subducted beneath the Caribbean and (- 130 km) left-lateral offset along the Cuilco- North American plates along the Middle Amer- Chixoy-Polochic system (CCP) since about 15 Ma ica Trench (MAT, Fig. l), whereas the boundary (Burkart, 1978, 1983; Burkart et al., 1987). Move- between the Caribbean and North American ment along the Jocotan and Motagua systems is plates is a complex set of left-lateral transform less well determined. They do not have a geomor- faults. The North America-Caribbean plate phic expression in the Miocene rocks that cover boundary includes the Motagua, Jocotan, and their western extension and do not produce size- Polochic systems in Honduras, Guatemala, and able offset in the Quatemary volcanic arc. In adjacent Mexico (Burkart, 1978; Schwartz et al., contrast, the eastern extension of the plate ~~-1951/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDlOO40-1951(94)0-0078-N AN Gulf of Tehuantepec Fig. I. Schematic representation of the plate tectonic arrangement of the Cocos, North America, and Caribbean plates. Open arrows represent relative motion directions. Those for northern Central America and centraf Mexico are indicative of bfock rotation Wrrutia-Fucugauchi and Biihnel, 1988; Burkart and Self, 1985). Arcuate strike-slip faults and north-south-trending grabens characterize Central Guatemala and Honduras. Central Chiapas is also characterized by a major WNW-ESE depression (Central Chiapas Depression, CCHDJ. Major faults with lateral displacement are shown with solid lines (dashed where uncertain). Normal faults have tick-marks on the down-thrown block. Thrust faults are also indicated on the eastern Chiapas Laramide fold belt. Miocene volcanic provinces of southern Mexico and Central America are indicated with a dark pattern and asterisks indicate main Quatema~ volcanic centers. Enlarged area: map of the southern Isthmus of Tehuantepec region showing paleomagnetic sites (1 through 9) and area1 exposure of Cenozoic volcanic and intrusive rocks. The dotted pattern inctudes undifferentiated pre-Cenozoic rocks of the Chiapas Batholith. Symbols on the map are: SC = Salina Cruz fault; CCP = Cuilco-Chixoy-Polochic fault system; MF = Motagua fault system; JF = Jocotan fault system; MPF = Malpaso fault system; CT = Cayman Trough. RX ~~l~a-ea~a et al. / Tec~ono~hysics 235 (1994) 339-346 341 boundary in the Cayman Trough indicates as trapped between the North America and Carib- much as 450 km of left-lateral displacement in bean plates. In the model proposed by Burkart the Neogene (Holcombe and Sharman, 1984) and and Self (1985), the tectonic and volcanic pattern a collective Cenozoic offset of N 1,000 km in northern Central America (the Chortis Block; (Rosencrantz et al., 1988). Burkart and Self (1985) Dengo, 1969) and adjacent Chiapas (south of the postulated that internal deformation (extension CCP system) is the result of eastward rotation and terrane rotation) along the transform bound- and differential movement of crustal blocks over ary in Guatemala and Honduras can account for the subduction-controlled zone of magma pro- a substantial part of the Neogene displacement duction (Fig. 1). The model of Guzman-Speziale observed offshore. Extensional structures (e.g., et al. (1989) predicts that the extensional tectonic pull-apart basins) and se~entation of the vol- regime and block rotations of central Guatemala canic arc appear to have been active at least should extend into westernmost Chiapas. during the Neogene (Schwartz et al., 1979; In contrast to seismic activity, which yields a Burkart, 1983; Carr, 1984). Early Cenozoic move- snapshot view of stress accumulation and relief ments along the Jocotan, Motagua, and Polochic along plate boundaries, paleomagnetic data may systems are, however, controversial. provide evidence of a longer lasting stress regime. The characteristics of the subduction zone and Paleomagnetic directions can be interpreted in associated magmatic arc show a marked contrast terms of past or present tectonic regimes. Discor- across the transform boundary. The region of the dant paleomagnetic directions of Cenozoic voi- Isthmus of Tehuantepec in southern Mexico ap- canic, plutonic, and sedimentary units of the Cen- pears as a zone of transition from shallow sub- tral Mexico Volcanic Belt have been interpreted duction dip angles beneath southwestern Mexico in terms of countercl~~ise rotations of smafl to steeper angles beneath Chiapas and Central crustal blocks in response to large scale left-lateral America. The volcanic activity shows a disconti- shear (shown schematically in Fig. 1) within a nuity from the well-defined, close-to-the-margin transtensional environment (Urrutia-Fucugauchi Central American arc parallel to the MAT, to the and BShnel, 1988). Regional stress and faulting in oblique and somewhat irregularly distributed pat- the volcanic belt appear to be related to complex tern of southern Mexico (Fig. 3). plate interactions and reorganization in the Pa- The location and nature of the place where cific region (Mammerickx and Klitgord, 1982). the three plates come together is also a matter of Interestingly, paleomagnetic data for Cretaceous debate. The present plate geometry and pIate through Tertiary rocks in southern Mexico ter- velocities indicate that a theoretical triple junc- ranes (south of the Central Mexico VoIcanic Belt) tion (trench-trench transform) between North including Early Cretaceous rocks overlying the America, Cocos and Caribbean should be unsta- Chiapas Batholith, suggest stability with respect ble. Thus past plate boundary configurations are to the craton. In fact, paleomagnetic data pre- difficult to incorporate in a tectonic model for the clude major tectonic movements other than lati- region. The evolution of the triple junction is tude-parallel motion (Urrutia-Fucugauchi et al., likely responsible for the complex deformation 1987; Bohnel et al., 1988, 1989; Urrutia- pattern in this area. Although most models have Fucugauchi, 1988; Guerrero et al., 1990). Paleo- assumed that the Polochic or Motagua faults, or magnetic data for the Chortis Block are scarce, both, intersect the Middle America Trench but Late Jurassic through Eocene data clearly (Burkart, 1983), a lineation of seismicity oriented show large-magnitude rotations with respect to northwest-southeast in central Chiapas (the MaI- southern Mexico accompanied by relative south- paso fault system) has been interpreted by Guz- wards motion (Gose, 1985). The paIeomagnetic man-Speziale et al. (1989) as a significant plate data from the Chortis Block are compatible with boundary, In the model of Guzman-Speziale et models that suggest it has been adjacent to south- al. (1989), the region of western Chiapas (and west Mexico since Early Cretaceous times (Pindell offshore) is a small wedge of the Caribbean plate and Barrett, 1990). In this study we report paleomagnetic data for 2. Gehgy and sampling Late Cenozoic volcanic and plutonic units ex- posed along the Pacific coastal plain in the south- The southeastern portion of the Pacific coastal eastern portion of the Isthmus of Tehuantepec. plain in the Tehuantepec Isthmus region is char- This locality is presently northeast of the pro- acterized by Miocene volcanic, subvolcanic, and jected intersection of the MAT and the CCP fault plutonic rocks (Damon and Montesinos, 1978; system, the most northern of the left-lateral fault Burkart, 1983) which in part intrude the late systems at the North America-Caribbean bound- Paleozoic Chiapas Batholith and the Mesozoic ary. Paleomagnetic samples were obtained from sedimentary cover (Fig. 1). These units represent Late Cenozoic rocks to examine whether tectonic the ancient location of a magmatic arc that ex- rotations such as proposed for northern Central tended to
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