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Carbonate detritus and mylonite zones in Guerrero, Mexico and northern Honduras: new evidence for detachment of the Chortis block from southern Mexico
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Journal of South American Earth Sciences, Vol. 11, No. 3, pp. 291±307, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0895-9811/98 $ - see front matter S0895-9811(98)00016-9
Carbonate detritus and mylonite zones in Guerrero, Mexico and northern Honduras: new evidence for detachment of the Chortis block from southern Mexico R.A. MILLS* 175, Water Front Drive, Montgomery, TX 77356, USA (Received December 1996; accepted April 1998) Abstract Ð Brecciated lower Cretaceous carbonates on a collapsed margin in Guerrero add support to the growing evidence for the Chortis block originating from, and later detaching from, southern Mexico. Fault blocks with over 2000 m of breccia have been measured, suggesting breakup of a carbonate platform due ®rst to magmatic uplift and later due to transcurrent faulting during detachment. There is less breccia on the Chortis block; however, magmatism may have been responsible for extensive block faulting which resulted in thick sections of well rounded limestone conglomerate that have been observed in Honduras. Mylonite scars may be evidence of basement rifting and are the basis for dividing the Xolapa complex, created by early Paleogene magmatism, into three sub-blocks. At about the same time Chortis may also have been broken into three subblocks by sinistral forces. A tectonic history is proposed that is based on published work by others but supported by data presented in this paper. # 1998 Elsevier Science Ltd. All rights reserved Resumen Ð Carbonato brecciado de edad Cretacico inferior en una margen hundimentada en Guerrero aumenta el sosteÂn de la evidencia para la teoria que el bloque Chortis fue orginado y despues separado de la parte sur de Mexico. Secciones falladas con maÂs de 2000 metros de caliza brecciada han sido medidas sugeriendo un rompimiento de una plataforma carbonata, primero debido a un levantamiento magmatico y despues a una fallada transcurrente durante la separacioÂn. Hay menos caliza brecciada sobre Chortis, sin embargo, actividad magmatica pudo ser la causa para la fallada extensiva la cuaÂl resulto en secciones gruesas de caliza conglomeradas de forma muy redonda que se han observado en Honduras. Rocas de mylonita podrian ser evidencia en la hendedura basamento y es la base el cuaÂl divide el complejo Xolapa, causada por Poleogene magmatica de edad temprana en tres sub-bloques. Casi al mismo tiempo Chortis tambieÂn pudo haber sido quebrado en tres sub-bloques por fuerzas sinistral. Una historia tectonica es propuesta que esta basada en trabajos por otros pero sostenida por datos presentados por este artõ culo. # 1998 Elsevier Science Ltd. All rights reserved
the Caribbean plate when South America separated from North America. This idea has been discussed by numerous authors including Gose and Sanchez Barreda (1981), Burkhart (1983), Azema et al. (1985), Gose (1985), Manton (1987), Pindell et al. (1988), Ross and Scotese (1988) and Finch and Dengo (1990). Considerable isotope chronology (Moran-Zenteno et al., 1990; Moran-Zenteno, 1992; Sedlock et al., 1993; Hermann et al. 1994; Schaaf et al. 1995) provided a basis for the timing of the detachment of Chortis from southern Mexico. Riller et al. (1992) described left lateral transtension in a prominent mylonite zone near Tierra Colorada in southern Guerrero.
INTRODUCTION Brecciated lower Cretaceous limestones were well exposed in fresh outcrops on the new toll highway linking Mexico City and Acapulco in 1994 and 1995. Since then many of the exposures have been covered with cement for erosion control. Prior to this cover, the author was able to measure several brecciated sections that individually were 2000 m thick. Conglomeratic and brecciated sections have been reported in northern Honduras (Mills et al., 1967; Southernwood, 1986; Manton, 1987). Noted in this paper is an outcrop on a rebuilt road in northern Honduras that exhibited over one hundred meters of severely contorted and brecciated Cretaceous shales and limestones.
It is suggested in this paper that the signi®cant brecciation noted above is evidence for the breakup of a common carbonate platform connecting southern Mexico and the Chortis block. Mylonitized shear zones recorded in Guerrero and in northern Honduras may illustrate how the basement terranes were oset into sub-blocks during the detachment of the Chortis block. This paper will also show the dierence between the carbonate breccia in Guerrero and the carbonate conglomerate in northern Honduras.
There is some similarity between Mesozoic stratigraphy in southern Mexico and northern Honduras and it has been speculated that the Chortis block, including northern Honduras, was once part of southern Mexico and then dislocated and rotated with * Tel: +1-409-582-4124; Fax: +1-409-582-4332. 291
Fig. 1. Geographic localities of southern Mexico and Central America. A, Acapulco; Ch, Chilpancingo; H, Hualtuco; I, Lago Izabal; L, La Luz gold mining district; P, Pinotepa Nacional; SF, San Francisco de la Paz; TC, Tierra Colorada; Y, Lago Yojoa; Z, Zihuatanejo; PV, Puerto Vallarta; G, Guatemala City, T, Tegucigalpa.
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Fig. 2. Generalized tectonic map of southern Mexico and northern Central America in recent time. Projection is Lambert azimuthal equal area. TMVB, Trans Mexico volcanic belt; AT, Acapulco trench; MAT, Middle America trench; SIT, Swan Island transform fault zone; NR, Nicaragua rise; CPF, Chixoy±Polochic fault zone; MJCAF, Motagua±Jocotan± Chamelecon±Aguan fault system; GF, Guayape fault zone; PF, Patuca fault zone; C, Colon block (lower Cretaceous carbonates); CAVB, Central America volcanic belt. See Fig. 1 for other symbols.
See Fig. 1 for the geography of southern Mexico and Central America. Included are place names used in this paper. Figure 2 illustrates the simpli®ed tectonic framework of southern Mexico and the Chortis block. While this study concentrates on the detachment of the northern coast of Chortis, it should be noted that Mills and Barton (1996) described tectonic events in the northeast part of Honduras which would have been on the southeast or seaward side of the Chortis block when it was attached to southern Mexico. Drag from the advancing Caribbean plate as early as late Cretaceous time may have dislocated the southeast side of Chortis including the Colon sub-block along a series of sinistral faults, the most prominent being the Patuca fault (Fig. 2).
RECONSTRUCTED GEOLOGIC SETTING Refer to Fig. 3 for a summary of stratigraphic and magmatic units in the Chilpancingo±Acapulco,
Guerrero, and northern Honduras areas. Although there are variations, the major units in both areas are quite similar. Basement rocks are the Las Ovejas (Guatemala± Honduras) (Finch and Dengo, 1990) and Acatlan (Guerrero state) (Riller et al., 1992), metamorphic complexes that are at the base of the Mixteca terrane (after Campa and Coney, 1983) in southern Mexico. Both complexes include a variety of meta-sedimentary and meta-volcanic rocks that have been radiometrically dated as Precambrian to late Paleozoic in age (Sedlock et al., 1993). On the basis of previously published reconstructions, Chortis is believed to have been an integral part of the Mixteca terrane until the end of Mesozoic time. The earliest Mesozoic rocks are dark clastic strata and thin limestones, early Jurassic in age, that are called the Chapolapa Formation in Guerrero (MoranZenteno et al., 1984) and the Agua Fria Formation in Honduras (Finch and Ritchie, 1985). These sediments were deposited in intramontane troughs and shallow
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Fig. 3. Generalized stratigraphic and magmatic events of Chilpancingo±Acapulco, Guerrero, and northern Honduras areas.
embayments. Following uplift and erosion during late Jurassic time, general subsidence throughout the Cretaceous period resulted in thick carbonate platforms, redbeds and thin-bedded limestones with organic shales. The lower Cretaceous thick limestones are called the Morelos Formation in Guerrero (Moran-Zenteno et al., 1984) and the Yojoa Group in Honduras (Mills et al., 1967). The upper Cretaceous rocks include the Balsas Group and the Mexcala Formation in Guerrero (Moran-Zenteno et al., 1984) and the Valle de Angeles Group in Honduras (Finch 1981). The Mixteca terrane was aected by arc magmatism throughout Cretaceous and Paleogene times. During the Paleogene the terrane was severely deformed and altered so that the Acatlan/Las Ovejas terranes were reconstituted as the Xolapa complex (Guerrero) and the Cacaguapa complex (Honduras) (Schaaf et al., 1995; Horne et al., 1976). During the Neogene, extensive volcanic sequences, mostly ignimbrites and other pyroclastic sections, covered parts of Guerrero and Honduras. MoranZenteno et al. (1996) describe the tectonic history, including extensive subduction erosion, along the southwest coast of Mexico since Oligocene time. Chilpancingo±Acapulco, Guerrero area Brecciation of lower Cretaceous carbonate strata. The geology of the Chilpancingo±Acapulco area (Fig. 4) was taken from the Mexican Carta Geologica (1981), Moran-Zenteno (1992), Riller et al. (1992), and reconnaissance ®eld mapping by the author. Note the ®eld station numbers and sections A±A', near
Mazatlan, and B±B', south of Tierra Colorada. The sections, to be supplied by the author on request, show close to 2000 m of limestone breccia measured in single fault blocks. Field studies suggest that over 80% of the exposed lower Cretaceous limestones in this area are brecciated to some degree. However, the brecciation is often interrupted with 20±50 m intervals of dark grey to black, thick limestones interbedded with thin black limey shales. Near Mazatlan, about 12 km south of Chilpancingo, the breccia is relatively undeformed and the depositional character can be determined. The breccia is a mixture of primary and secondary clasts, and in some exposures there are tertiary clasts that include secondary clasts. The size of the clasts vary from 1 mm to boulder size, but generally they are 5± 20 cm in diameter. The clasts are mostly subangular, indicating some transport before ®nal deposition. The matrix is dark grey to reddish brown, crystalline limestone containing microbreccia. In some samples the matrix is oxidized and reddish brown in color. The limestone breccia is not to be confused with limestone conglomerate where the clasts are usually wellrounded and are in a sandstone matrix. The brecciated section represents a basin margin or trough environment where a thick carbonate platform was uplifted and ruptured by arc magmatism. Subsequently it slumped and rock falls, with debris ¯ow and grain ¯ow, resulted. See Cook and Mullins (1983) for a detailed description of rock types around carbonate margins. In contrast to a conglomeratic section, the breccia was deposited near its source and
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Fig. 4. Generalized geologic map of Chilpancingo±Acapulco, Guerrero area. Plutonic and volcanic rocks: Tgd,gr, Tertiary granodiorites, granites; Kgd, Cretaceous granodiorites. Sedimentary rocks: Q, Quaternary alluvium; Ti, lower Tertiary clastics; Ks, upper Cretaceous limestone and limestone conglomerate; Ki, lower Cretaceous limestone and limestone breccia; Ji, lower Jurassic clastics with thin limestones. Metamorphic rocks: My, mylonite outcrops; Xo, early Paleogene Xolapa complex; Pa, Paleozoic Acatlan complex. Strike and dip in degrees for limestone breccia is noted, small triangles are geologic stations, squares are locations of concordant radiometric dates in Ma (see MoraÂn-Zenteno, 1992) for isotopes and method. Cross sections A±A' and B±B' can be supplied by the author on request.
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Plate 1. View of 600 m of massive, lower Cretaceous limestone breccia on the Rio Papagayo near Tierra Colorada. View is looking north.
then reworked one or more times by repeated uplifts and fault deformation. The fault block near Tierra Colorada is a remnant of the brecciated platform that was disrupted by the left lateral oset that was mapped by Riller et al. (1992). The block is composed of bedded limestone breccia, dipping north at about 40 degrees. The section exposed along the Rio Papagayo appears undisturbed as seen in Plate 1; however, on the outcrop the limestone is intensely brecciated. Most of the section exhibits secondary to tertiary brecciation that has subsequently been compressed (see Plate 2). The deformation near the detachment was particularly intense, as indicated by shattering and rebrecciation of the primary breccia and later in-®lling with calcite veins, sometimes covering 40% of the rock. The fractures follow no pattern and are hairline to 50 cm wide. They are ®lled with white to brownish (oxidized) amorphous to crystalline calcite. In places there are a bioherms with algal, rudistid and other skeletal debris that has been deformed. Thin sections show the skeletal material to be completely recrystallized. A thin section at station 128, near the bioherm, shows primary, secondary and tertiary clasts on a microscopic scale. Even the matrix between clasts is composed of small brecciated clasts. The thin section can be supplied by the author on request. The lower Cretaceous age of the limestone clasts was partially con®rmed with Orbitulina texana fragments found in one thin section. Mylonitization on the surface of the Xolapa complex There is a signi®cant absence of limestone south of the left lateral fault zone mapped by Riller et al.
(1992), near Tierra Colorada. The fault zone is shown on Fig. 4. The Xolapa complex, however, contains numerous mylonitic scars suggesting that basement rock rode over the Xolapa in this region. It is speculated that as the basement rock of the Chortis block detached, it may have broken the Xolapa into several sub-blocks. These sub-blocks were dragged over each other by the left lateral movement. Evidence for this idea is largely from the topography of the Xopala complex. There appears to be three sub-blocks: the Zihuatanejo to the west, Acapulco in the center, and Pinotepa to the east. Following this theory the Pinotepa block rode over the Acapulco block leaving the mylonitic scars mentioned above. Moran-Zenteno (1992) describes several exposures of mylonite in the Tierra Colorada±Acapulco±Rio Papagayo area. The author noted some new exposures on the Mexico City±Acapulco toll road and on the Rio Papagayo. All of these exposures are shown in Fig. 4. One exposure at station 96 on the Rio Papagayo is illustrated in Plate 3. A thin section of a dark band of mylonite at station 37 near Tierra Colorada shows pronounced linearity and relic clastic crystals. The original rock may have been an orthogneiss rich in mica. The strike of the mylonite fabric at most of the localities is generally east±west. Northern Honduras Deformation of the Cretaceous carbonates. A generalized tectonic map of northern Honduras, a large part of the Chortis block, is shown on Fig. 5. This was taken from the Instituto Geogra®co Nacional Mapa Geologico de Honduras, segunda edicion, 1991,
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Plate 2. Near Tierra Colorada at station 126. See Figs. 4 and 5 for location. Primary, secondary and tertiary limestone breccia that has been deformed. Note compression in front of hammer.
compiled by M.T. Kozuch, Manton (1987) and ®eld mapping by the author. Thirty meters or more of lower Cretaceous breccias were observed at the south end of Lago Yojoa and along the Chamelecon fault. See Fig. 5 for sample locations. The author can provide descriptions on request.
A well-exposed section of deformed carbonates was measured 4.5 km northwest of San Francisco de la Paz in the department of Olancho. This was a new exposure in 1994 due to the improvement of the Trujillo±Juticalpa road. See Fig. 6 and Plates 4 and 5. Exposed are uplifted, faulted, upper Cretaceous Valle de Angeles limestones and shales that are dated from
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Plate 3. Mylonite outcrop about 10 km inland of the mouth of Rio Papagayo at station 96. See Fig. 4 for location.
®eld relationships. From overlapping photos, over 125 m were measured. At the core of the section is a weathered granitic intrusion that is ¯anked by a series of transtensional and transpressional faults that in one place expose down-thrown upper Valle de Angeles redbeds. The granite plug and associated faults are probably related to a previously unmapped fault zone that in part parallels the Guayape fault zone, see Fig. 5 (Finch and Ritchie, 1991). The whole section has been mega-brecciated by the deformation. It appears that most of the brecciation seen in Honduras is due to deformation rather than slumping of a platform margin. There is less evidence for early Cretaceous brecciation in Honduras than seen in Guerrero, Mexico. Most of the detrital limestones in Honduras are Ilama conglomerates of the upper Cretaceous Valle de Angeles group, after Mills and Barton (1996). The Ilama, originally described by Mills (1967), and later by Southernwood (1986), is composed of rounded limestone and volcanic clasts supported by a red to brown sandstone matrix. The conglomerates are probably due to block faulting of the Yojoa group carbonates before Chortis detached from southern Mexico. Widespread brecciation seen in Guerrero occurred later in Paleogene time during the detachment. Mylonitization on the Cacaguapa surface Manton and Manton (1989) describe a mylonitized shear zone on the Rio Cangrejal south of La Ceiba on the Caribbean coast that may be related to the La Ceiba fault system, which in turn may be an extension of the Jocotan fault zone on the Guatemala± Honduras border. For an excellent photograph of the Rio Cangrejal mylonite zone see the frontispiece of Tectonics 9(2), 1990.
Simonson (1977) describes a 202 kilometer exposure of granitic and tonalitic mylonite near El Provenir in the department of Francisco Morazan. The author speculates this zone may be related to the fault zone near San Francisco de la Paz (Fig. 5). Surprisingly there are few recorded exposures of mylonite along the suture zone between the Chortis block and the Maya block. Ophiolitic sequences, however, are common in this area, Finch and Dengo (1990). TECTONIC SUMMARY The tectonic events of the two regions are determined from previously published work as noted, and speculation by the author that is partially supported by the carbonate brecciation and distribution of mylonite zones. Late Cretaceous (Figs 7 and 10) The thick Early Cretaceous carbonate platforms were disrupted by an extensive magmatic arc between 104 and 65 Ma that extended along the Paci®c coast from Puerto Vallarta to the Acapulco area and then across northern Chortis where there was extensive block faulting. See Schaaf et al., (1995) and dates in Figs 4 and 5 that are shown next to small squares. Over 350 m of redbeds with rounded limestone conglomerates were deposited in block faulted valleys. Early Paleogene (Figs 7 and 10) The Cretaceous magmatism created a weak zone between the Acatlan complex and the Las Ovejas complex. A second period of intense igneous activity between 60 and 50 Ma along this weak zone formed
Fig. 5. Generalized tectonic map of northern Honduras. Heavy lines, major transcurrent faults; FZ, fault zone; small triangles, geologic stations; small squares, concordant radiometric dates in Ma. See Sedlock et al. (1993) and Manton (1987) for isotopes and methods.
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R. A. Mills Fig. 6. Measured section from photographs near San Francisco de la Paz, station 48. Section illustrates Valle de Angeles limestones and shales that have been severely deformed and mega-brecciated due to a granitic intrusion and related transtension faulting. Note locations of plates 4 and 5.
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the Xolapa and Cacaguapa complexes and at the same time severely ruptured and brecciated the platform carbonates, dumping them into several troughs, one of which was between Mazatlan and Tierra Colorada in Guerrero, and another one may have been along the Chamelecon fault zone in northern Honduras. Late Eocene±Early Oligocene (Figs 8 and 10) Schaaf et al. (1995) describe radiometric dates generally decreasing from 104 Ma near Puerto Vallarta to 21 Ma near Huatulco, a distance of 1100 km. This apparent migration of magmatism was due to changes of dip and velocity of the subducting Farallon plate. Hermann et al. (1994), also suggest that the Farallon subduction was oblique to the continent during the Paleogene. The Farallon subduction was probably related to the eastward advance of the Caribbean plate (Ross and Scotese, 1988). By 43 my, the vector of subduction (stage pole) was about 53 degrees azimuth (Engebretson et al., 1985). Therefore from Late Eocene to Early Oligocene time the subduction was to the northeast and it detached and carried eastward the various subblocks of Xolapa and Chortis. Magmatism also followed this eastward trend. Late Oligocene±Quaternary Figures 8 and 9 show the sub-blocks of the Xolapa and Chortis blocks being detached, riding over one another to some degree, and moving eastward. By Late Oligocene time the three Xolapa blocks were more or less in their present positions. The Chortis block, however, had left the Xolapa sub-blocks behind and was translating with the Caribbean plate. The actual route of the Chortis block is controversial. For example, with paleomagnetic data, Gose (1985) describes a 100 degree clockwise rotation and then a 100 degree counter-clockwise rotation before the northern margin of Chortis sutured to the Maya block in Late Oligocene or Early Miocene time. Field data presented in this paper suggests a post Cretaceous age for the break-up of Xolapa and Chortis; however, previously published radiometric data (Schaaf et al., 1995) narrow the time to between the late Eocene and the Late Oligocene, i.e., 43±24 Ma. During Neogene time, the vector of subduction was about 14 degrees azimuth (Pindell et al., 1988), or nearly perpendicular to the coast. This head-on subduction formed the younger plutons to the east around Huatulco. During Quaternary time, the subduction created the Acapulco and Middle America trenches with attendant volcanism responsible for continental accretion and the spectacular volcanic chain along the Paci®c coast of Central America (see Fig. 2 and Finch and Dengo 1990).
Carbonate detritus and mylonite zones in Guerrero, Mexico
Plate 4. A,B. Near San Francisco de la Paz, department of Olancho station 48. See Fig. 6 for location. Severely deformed and brecciated upper Cretaceous Valle de Angeles carbonates.
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Plate 5. A. Near San Francisco de la Paz, station 48. Complex section of lower Valle de Angeles carbonates showing altered breccia thrust over vertical beds of limestone and limestone breccia. B. Near San Francisco de la Paz, station 48. Highly weathered granite plug intruding lower Valle de Angeles brecciated carbonates.
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Fig. 7. Paleotectonic mapÐlate Cretaceous±early Paleogene time. Projection is Lambert azimuthal equal area. See Fig. 1 for geographic localities. Small oblong areas are approximate locations of late Cretaceous igneous intrusions. Note vector of subduction for the Farallon plate.
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Fig. 8. Paleotectonic mapÐlate Eocene±early Oligocene time. Same projection as Fig. 7. See Fig. 1 for geographic localities. Note location for cross section C±C' that is represented on Fig. 10.
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Fig. 9. Paleotectonic mapÐlate Oligocene time. Same projection as Fig. 8. NR, Nicaragua Rise; J, Jamaica. See Fig. 1 for geographic localities.
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Fig. 10. Diagrammatic sections along cross section C±C' shown on Fig. 8. There are six time sections for the Triassic, Jurassic, late Cretaceous, early Paleogene, late Eocene to early Oligocene, and late Oligocene. T/A, transcurrent fault; Tblock toward reader; A-block away from reader.
Chortis may have broken into sub-blocks that were oset several times by sinistral and dextral faults (Gordon and Muelberger 1994). Evidence for the subblocks includes previously described fault-related mylonite zones and the progressive easterly oset of the Honduras Caribbean coast along major fault zones (Figs 2 and 9). Although the Honduras fault zones are not well understood, it is possible they were oset by sinistral drag associated with the eastern translation of the Caribbean plate. The timing of the oset is not known. It may be the oldest faults are on the east side of Chortis while the younger ones are to the west.
CONCLUSIONS Field studies by the author in Guerrero and northern Honduras revealed similar zones of brecciation and mylonitization that are related to fault zones connected to the separation of Chortis from southern Mexico. It is suggested that the detachment of Chortis may have ruptured the Xolapa complex into three sub-blocks. These blocks rode over each other progressively to the east, leaving mylonitic scars. At the same time, the Cretaceous carbonate platform, covering the joint terrane, was severely deformed and col-
Carbonate detritus and mylonite zones in Guerrero, Mexico lapsed in southern Guerrero leaving thick accumulations of breccia. The absence of limestone and breccia south of the thick exposures, near Tierra Colorada, indicates that the Chortis block rotated over the Xolapa complex, leaving it barren. Chortis probably was broken into sub-blocks by sinistral forces related to the rotation of the Caribbean plate. Detrital limestone on Chortis is largely molasse type conglomerates ®lling valleys created by extensive block faulting in late Cretaceous time. Acknowledgements Ð The author is indebted to Max Suter and Richard Finch for original inspiration to start the study; Mark Gordon, who supplied many valuable references; Luis SanchezBarreda, who provided valuable contacts in Acapulco; Dante Moran-Zenteno, who personally, and with his sta, provided abundant data, suggestions and encouragement; and ®nally to my son, John, and my wife, Nohemi, who spent long days assisting with the ®eld work and preparation of the manuscript.
REFERENCES Azema, J., Biju-Dival, B., Bizon, J.J., Carfantan, J.C., Mascle, A. and Tardy, M. (1985) Le Honduras (Amerique centrale nucleaire) et le block d'Oaxaca (sud du Mexique): duex ensembles comparables du continent Nord-Americain separes parle jeu decrochant senestre des failles due systeme Polochic±Motagua. In Geodynamique des Caraibes, ed. Mascle A., 427±438. Editions Technip, Paris. Burkart, B. (1983) Neogene North American±Caribbean plate boundary across northern Central America: oset along the Polochic fault. Tectonophysics 99, 251±270. Campa, M.F. and Coney, P.J. (1983) Tectono-stratigraphic terranes and mineral resource distributions in Mexico. Canadian Journal of Earth Science 26, 1040±1051. Cook, H.E. and Mullins, H.T. (1983) Carbonate depositional environments: Basin margin. In American Association of Petroleum Geologists, eds. Scholle P.A., Bebout D.G. and Moore C.H. Memoir 33. Engebretson, A.C., Cox, A. and Gordon, R.G. (1992) Relative motions between oceanic and continental plates in the Paci®c Basin. Geological Society of American Special Paper 206, 59 pp.. Finch, R.C. (1981) Mesozoic stratigraphy of central Honduras. American Association of Petrologists Bulletin 65, 1320±1333. Finch, R.C. and Ritchie, A.W. (1985) Widespread Jurassic strata on the Chortis block of the Caribbean plate (abs.). Geological Society of America Abstracts with Programs 17, 700±701. Finch, R.C. and Dengo, G. (1990) Noam-Carib plate boundary in Guatemala: a Cretaceous suture zone reactivated as a Neogene transform fault. In Geological Society of America 1990 Annual Meeting. Field Trip 17. Finch, R.C. and Ritchie, A.W. (1991) The Guayape fault system, Honduras, Central America. Journal of South American Earth Sciences 4, 43±60. Gordon, M.B. and Muehlberger, W.R. (1994) Rotation of the Chorlis block causes dextral slip on the Guayape fault. Tectronics, 13(4), 858±872. Gose, W.A. and Sanchez-Barreda, L.A. (1985) Paleomagnetic results from southern Mexico. Geo®sica Internacional (Mexico) 20, 163±175. Gose, W.A. (1985) Paleomagnetic results from Honduras and their bearing on Caribbean tectonics. Tectonics 4(6), 565±585.
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Hermann, U.R., Nelson, B.K. and Ratschacher, L. (1994) The origin of a terrane: U/Pb zircon geochronology and tectonic evolution of the Xolapa complex (southern Mexico). Tectonics 13, 455±474. Horne, G.S., Clark, G.S. and Pushkar, P. (1976) Pre-Cretaceous rocks of northwestern Honduras: Basement terrane in Sierra de Omoa. American Association of Petroleum Geologists 60, 566± 583. Manton, W.I. (1987) Tectonic interpretation of the morphology of Honduras. Tectonics 6, 633±651. Manton, R.S. and Manton, W.I. (1989) A major Late Cretaceous dextral strike-slip zone on the north coast of Honduras. Geological Society of America Abstracts with Programs 21, 203± 204. Mills, R.A., Hugh, K.E., Feray, D.F. and Swolfs, H.C. (1967) Mesozoic stratigraphy of Honduras. American Association of Petroleum Geologists Bulletin 51, 1711±1786. Mills, R.A. and Barton, R. (1996) Geology of the Ahuas area in the Mosquitia basin of Honduras: a preliminary report. American Association of Petroleum Geologists Bulletin 80, 1627±1640. Moran-Zenteno, D.J. () Geologia de la Republica Mexicana. Instituto Nacional de Estadistica Geogra®a e Informatica, Mexico, 88 pp. Moran-Zenteno, D.J., Kohler, H., Von Drach, V. and Schaaf, P. (1990) The geological evolution of the Xolapa terrane, southern Mexico, as inferred from Rb±Sr and Sm±Nd isotopic data. In Geowissencschaftliches Lateinmerika-Kolloquium (Munchen, FRG, November 1990), 40 p. Abstracts. Moran-Zenteno, D.J. (1992) Investigaciones isotopicas de RB±Sr y Sm±Nd en rocas cristalinas de la region de Tierra Colorada± Acapulco±Cruz Grande, Estado de Guerrero. Thesis. Universidad Nacional Autonoma de Mexico. Moran-Zenteno, D.J., Corona-Chavez, P. and Tolson, G. (1996) Uplift and subduction erosion in southwestern Mexico since the Oligocene: pluton geobarometry constraints. Earth and Planetary Science Letters 141, 51±65. Pindell, J.L., Cande, S.C., Pitman III, W.C., Rowley, D.B., Dewey, J.F., Labrecque, J. and Haxby, W. (1988) A plate kinematic framework for models of Caribbean evolution. Tectonophysics 155, 121±138. Riller, U., Ratschbacher, L. and Frisch, W. (1992) Left-lateral transtension along the Tierra Colorada deformation zone, northern margin of the Xolapa magmatic arc of southern Mexico. Journal of South American Sciences 5(34), 237±249. Ross, M.I. and Scotese, C.R. (1988) A hierarchical tectonic model of the Gulf of Mexico and Caribbean region. Tectonophysics 155, 139±168. Sedlock, R.L., Ortega-Gutierrez and Speed, R.C. (1993) Tectonostratigraphic terranes and tectonic evolution of Mexico. Geological Society of America Special Paper 278. Schaaf, P., Moran-Zenteno, D.J., Hernandez-Bernal, M., SolisPichardo, G., Tolson, G. and Kohler, H. (1995) Paleogene continental margin truncation in southwestern Mexico: geochronological evidence. Tectonics 14(5), 1339±1350. Simonson, B.M. (1977) Geology of the El Porvenir quadrangle, Honduras, Central America, Open-File Report edn, 84. Teguicigalpa, Departamento de Geologia e Hidrogra®a, Intituto Geogra®co Nacional de Honduras. Southernwood, R. (1986) Late Cretaceous limestone clast conglomerates of Honduras. Unpublished MS thesis. University of Texas at Dallas, p. 298.
Journal of South American Earth Sciences, Vol. 11, No. 3, pp. 291±307, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0895-9811/98 $ - see front matter S0895-9811(98)00016-9
Carbonate detritus and mylonite zones in Guerrero, Mexico and northern Honduras: new evidence for detachment of the Chortis block from southern Mexico R.A. MILLS* 175, Water Front Drive, Montgomery, TX 77356, USA (Received December 1996; accepted April 1998) Abstract Ð Brecciated lower Cretaceous carbonates on a collapsed margin in Guerrero add support to the growing evidence for the Chortis block originating from, and later detaching from, southern Mexico. Fault blocks with over 2000 m of breccia have been measured, suggesting breakup of a carbonate platform due ®rst to magmatic uplift and later due to transcurrent faulting during detachment. There is less breccia on the Chortis block; however, magmatism may have been responsible for extensive block faulting which resulted in thick sections of well rounded limestone conglomerate that have been observed in Honduras. Mylonite scars may be evidence of basement rifting and are the basis for dividing the Xolapa complex, created by early Paleogene magmatism, into three sub-blocks. At about the same time Chortis may also have been broken into three subblocks by sinistral forces. A tectonic history is proposed that is based on published work by others but supported by data presented in this paper. # 1998 Elsevier Science Ltd. All rights reserved Resumen Ð Carbonato brecciado de edad Cretacico inferior en una margen hundimentada en Guerrero aumenta el sosteÂn de la evidencia para la teoria que el bloque Chortis fue orginado y despues separado de la parte sur de Mexico. Secciones falladas con maÂs de 2000 metros de caliza brecciada han sido medidas sugeriendo un rompimiento de una plataforma carbonata, primero debido a un levantamiento magmatico y despues a una fallada transcurrente durante la separacioÂn. Hay menos caliza brecciada sobre Chortis, sin embargo, actividad magmatica pudo ser la causa para la fallada extensiva la cuaÂl resulto en secciones gruesas de caliza conglomeradas de forma muy redonda que se han observado en Honduras. Rocas de mylonita podrian ser evidencia en la hendedura basamento y es la base el cuaÂl divide el complejo Xolapa, causada por Poleogene magmatica de edad temprana en tres sub-bloques. Casi al mismo tiempo Chortis tambieÂn pudo haber sido quebrado en tres sub-bloques por fuerzas sinistral. Una historia tectonica es propuesta que esta basada en trabajos por otros pero sostenida por datos presentados por este artõ culo. # 1998 Elsevier Science Ltd. All rights reserved
the Caribbean plate when South America separated from North America. This idea has been discussed by numerous authors including Gose and Sanchez Barreda (1981), Burkhart (1983), Azema et al. (1985), Gose (1985), Manton (1987), Pindell et al. (1988), Ross and Scotese (1988) and Finch and Dengo (1990). Considerable isotope chronology (Moran-Zenteno et al., 1990; Moran-Zenteno, 1992; Sedlock et al., 1993; Hermann et al. 1994; Schaaf et al. 1995) provided a basis for the timing of the detachment of Chortis from southern Mexico. Riller et al. (1992) described left lateral transtension in a prominent mylonite zone near Tierra Colorada in southern Guerrero.
INTRODUCTION Brecciated lower Cretaceous limestones were well exposed in fresh outcrops on the new toll highway linking Mexico City and Acapulco in 1994 and 1995. Since then many of the exposures have been covered with cement for erosion control. Prior to this cover, the author was able to measure several brecciated sections that individually were 2000 m thick. Conglomeratic and brecciated sections have been reported in northern Honduras (Mills et al., 1967; Southernwood, 1986; Manton, 1987). Noted in this paper is an outcrop on a rebuilt road in northern Honduras that exhibited over one hundred meters of severely contorted and brecciated Cretaceous shales and limestones.
It is suggested in this paper that the signi®cant brecciation noted above is evidence for the breakup of a common carbonate platform connecting southern Mexico and the Chortis block. Mylonitized shear zones recorded in Guerrero and in northern Honduras may illustrate how the basement terranes were oset into sub-blocks during the detachment of the Chortis block. This paper will also show the dierence between the carbonate breccia in Guerrero and the carbonate conglomerate in northern Honduras.
There is some similarity between Mesozoic stratigraphy in southern Mexico and northern Honduras and it has been speculated that the Chortis block, including northern Honduras, was once part of southern Mexico and then dislocated and rotated with * Tel: +1-409-582-4124; Fax: +1-409-582-4332. 291
Fig. 1. Geographic localities of southern Mexico and Central America. A, Acapulco; Ch, Chilpancingo; H, Hualtuco; I, Lago Izabal; L, La Luz gold mining district; P, Pinotepa Nacional; SF, San Francisco de la Paz; TC, Tierra Colorada; Y, Lago Yojoa; Z, Zihuatanejo; PV, Puerto Vallarta; G, Guatemala City, T, Tegucigalpa.
292 R. A. Mills
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Fig. 2. Generalized tectonic map of southern Mexico and northern Central America in recent time. Projection is Lambert azimuthal equal area. TMVB, Trans Mexico volcanic belt; AT, Acapulco trench; MAT, Middle America trench; SIT, Swan Island transform fault zone; NR, Nicaragua rise; CPF, Chixoy±Polochic fault zone; MJCAF, Motagua±Jocotan± Chamelecon±Aguan fault system; GF, Guayape fault zone; PF, Patuca fault zone; C, Colon block (lower Cretaceous carbonates); CAVB, Central America volcanic belt. See Fig. 1 for other symbols.
See Fig. 1 for the geography of southern Mexico and Central America. Included are place names used in this paper. Figure 2 illustrates the simpli®ed tectonic framework of southern Mexico and the Chortis block. While this study concentrates on the detachment of the northern coast of Chortis, it should be noted that Mills and Barton (1996) described tectonic events in the northeast part of Honduras which would have been on the southeast or seaward side of the Chortis block when it was attached to southern Mexico. Drag from the advancing Caribbean plate as early as late Cretaceous time may have dislocated the southeast side of Chortis including the Colon sub-block along a series of sinistral faults, the most prominent being the Patuca fault (Fig. 2).
RECONSTRUCTED GEOLOGIC SETTING Refer to Fig. 3 for a summary of stratigraphic and magmatic units in the Chilpancingo±Acapulco,
Guerrero, and northern Honduras areas. Although there are variations, the major units in both areas are quite similar. Basement rocks are the Las Ovejas (Guatemala± Honduras) (Finch and Dengo, 1990) and Acatlan (Guerrero state) (Riller et al., 1992), metamorphic complexes that are at the base of the Mixteca terrane (after Campa and Coney, 1983) in southern Mexico. Both complexes include a variety of meta-sedimentary and meta-volcanic rocks that have been radiometrically dated as Precambrian to late Paleozoic in age (Sedlock et al., 1993). On the basis of previously published reconstructions, Chortis is believed to have been an integral part of the Mixteca terrane until the end of Mesozoic time. The earliest Mesozoic rocks are dark clastic strata and thin limestones, early Jurassic in age, that are called the Chapolapa Formation in Guerrero (MoranZenteno et al., 1984) and the Agua Fria Formation in Honduras (Finch and Ritchie, 1985). These sediments were deposited in intramontane troughs and shallow
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Fig. 3. Generalized stratigraphic and magmatic events of Chilpancingo±Acapulco, Guerrero, and northern Honduras areas.
embayments. Following uplift and erosion during late Jurassic time, general subsidence throughout the Cretaceous period resulted in thick carbonate platforms, redbeds and thin-bedded limestones with organic shales. The lower Cretaceous thick limestones are called the Morelos Formation in Guerrero (Moran-Zenteno et al., 1984) and the Yojoa Group in Honduras (Mills et al., 1967). The upper Cretaceous rocks include the Balsas Group and the Mexcala Formation in Guerrero (Moran-Zenteno et al., 1984) and the Valle de Angeles Group in Honduras (Finch 1981). The Mixteca terrane was aected by arc magmatism throughout Cretaceous and Paleogene times. During the Paleogene the terrane was severely deformed and altered so that the Acatlan/Las Ovejas terranes were reconstituted as the Xolapa complex (Guerrero) and the Cacaguapa complex (Honduras) (Schaaf et al., 1995; Horne et al., 1976). During the Neogene, extensive volcanic sequences, mostly ignimbrites and other pyroclastic sections, covered parts of Guerrero and Honduras. MoranZenteno et al. (1996) describe the tectonic history, including extensive subduction erosion, along the southwest coast of Mexico since Oligocene time. Chilpancingo±Acapulco, Guerrero area Brecciation of lower Cretaceous carbonate strata. The geology of the Chilpancingo±Acapulco area (Fig. 4) was taken from the Mexican Carta Geologica (1981), Moran-Zenteno (1992), Riller et al. (1992), and reconnaissance ®eld mapping by the author. Note the ®eld station numbers and sections A±A', near
Mazatlan, and B±B', south of Tierra Colorada. The sections, to be supplied by the author on request, show close to 2000 m of limestone breccia measured in single fault blocks. Field studies suggest that over 80% of the exposed lower Cretaceous limestones in this area are brecciated to some degree. However, the brecciation is often interrupted with 20±50 m intervals of dark grey to black, thick limestones interbedded with thin black limey shales. Near Mazatlan, about 12 km south of Chilpancingo, the breccia is relatively undeformed and the depositional character can be determined. The breccia is a mixture of primary and secondary clasts, and in some exposures there are tertiary clasts that include secondary clasts. The size of the clasts vary from 1 mm to boulder size, but generally they are 5± 20 cm in diameter. The clasts are mostly subangular, indicating some transport before ®nal deposition. The matrix is dark grey to reddish brown, crystalline limestone containing microbreccia. In some samples the matrix is oxidized and reddish brown in color. The limestone breccia is not to be confused with limestone conglomerate where the clasts are usually wellrounded and are in a sandstone matrix. The brecciated section represents a basin margin or trough environment where a thick carbonate platform was uplifted and ruptured by arc magmatism. Subsequently it slumped and rock falls, with debris ¯ow and grain ¯ow, resulted. See Cook and Mullins (1983) for a detailed description of rock types around carbonate margins. In contrast to a conglomeratic section, the breccia was deposited near its source and
Carbonate detritus and mylonite zones in Guerrero, Mexico
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Fig. 4. Generalized geologic map of Chilpancingo±Acapulco, Guerrero area. Plutonic and volcanic rocks: Tgd,gr, Tertiary granodiorites, granites; Kgd, Cretaceous granodiorites. Sedimentary rocks: Q, Quaternary alluvium; Ti, lower Tertiary clastics; Ks, upper Cretaceous limestone and limestone conglomerate; Ki, lower Cretaceous limestone and limestone breccia; Ji, lower Jurassic clastics with thin limestones. Metamorphic rocks: My, mylonite outcrops; Xo, early Paleogene Xolapa complex; Pa, Paleozoic Acatlan complex. Strike and dip in degrees for limestone breccia is noted, small triangles are geologic stations, squares are locations of concordant radiometric dates in Ma (see MoraÂn-Zenteno, 1992) for isotopes and method. Cross sections A±A' and B±B' can be supplied by the author on request.
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Plate 1. View of 600 m of massive, lower Cretaceous limestone breccia on the Rio Papagayo near Tierra Colorada. View is looking north.
then reworked one or more times by repeated uplifts and fault deformation. The fault block near Tierra Colorada is a remnant of the brecciated platform that was disrupted by the left lateral oset that was mapped by Riller et al. (1992). The block is composed of bedded limestone breccia, dipping north at about 40 degrees. The section exposed along the Rio Papagayo appears undisturbed as seen in Plate 1; however, on the outcrop the limestone is intensely brecciated. Most of the section exhibits secondary to tertiary brecciation that has subsequently been compressed (see Plate 2). The deformation near the detachment was particularly intense, as indicated by shattering and rebrecciation of the primary breccia and later in-®lling with calcite veins, sometimes covering 40% of the rock. The fractures follow no pattern and are hairline to 50 cm wide. They are ®lled with white to brownish (oxidized) amorphous to crystalline calcite. In places there are a bioherms with algal, rudistid and other skeletal debris that has been deformed. Thin sections show the skeletal material to be completely recrystallized. A thin section at station 128, near the bioherm, shows primary, secondary and tertiary clasts on a microscopic scale. Even the matrix between clasts is composed of small brecciated clasts. The thin section can be supplied by the author on request. The lower Cretaceous age of the limestone clasts was partially con®rmed with Orbitulina texana fragments found in one thin section. Mylonitization on the surface of the Xolapa complex There is a signi®cant absence of limestone south of the left lateral fault zone mapped by Riller et al.
(1992), near Tierra Colorada. The fault zone is shown on Fig. 4. The Xolapa complex, however, contains numerous mylonitic scars suggesting that basement rock rode over the Xolapa in this region. It is speculated that as the basement rock of the Chortis block detached, it may have broken the Xolapa into several sub-blocks. These sub-blocks were dragged over each other by the left lateral movement. Evidence for this idea is largely from the topography of the Xopala complex. There appears to be three sub-blocks: the Zihuatanejo to the west, Acapulco in the center, and Pinotepa to the east. Following this theory the Pinotepa block rode over the Acapulco block leaving the mylonitic scars mentioned above. Moran-Zenteno (1992) describes several exposures of mylonite in the Tierra Colorada±Acapulco±Rio Papagayo area. The author noted some new exposures on the Mexico City±Acapulco toll road and on the Rio Papagayo. All of these exposures are shown in Fig. 4. One exposure at station 96 on the Rio Papagayo is illustrated in Plate 3. A thin section of a dark band of mylonite at station 37 near Tierra Colorada shows pronounced linearity and relic clastic crystals. The original rock may have been an orthogneiss rich in mica. The strike of the mylonite fabric at most of the localities is generally east±west. Northern Honduras Deformation of the Cretaceous carbonates. A generalized tectonic map of northern Honduras, a large part of the Chortis block, is shown on Fig. 5. This was taken from the Instituto Geogra®co Nacional Mapa Geologico de Honduras, segunda edicion, 1991,
Carbonate detritus and mylonite zones in Guerrero, Mexico
297
Plate 2. Near Tierra Colorada at station 126. See Figs. 4 and 5 for location. Primary, secondary and tertiary limestone breccia that has been deformed. Note compression in front of hammer.
compiled by M.T. Kozuch, Manton (1987) and ®eld mapping by the author. Thirty meters or more of lower Cretaceous breccias were observed at the south end of Lago Yojoa and along the Chamelecon fault. See Fig. 5 for sample locations. The author can provide descriptions on request.
A well-exposed section of deformed carbonates was measured 4.5 km northwest of San Francisco de la Paz in the department of Olancho. This was a new exposure in 1994 due to the improvement of the Trujillo±Juticalpa road. See Fig. 6 and Plates 4 and 5. Exposed are uplifted, faulted, upper Cretaceous Valle de Angeles limestones and shales that are dated from
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Plate 3. Mylonite outcrop about 10 km inland of the mouth of Rio Papagayo at station 96. See Fig. 4 for location.
®eld relationships. From overlapping photos, over 125 m were measured. At the core of the section is a weathered granitic intrusion that is ¯anked by a series of transtensional and transpressional faults that in one place expose down-thrown upper Valle de Angeles redbeds. The granite plug and associated faults are probably related to a previously unmapped fault zone that in part parallels the Guayape fault zone, see Fig. 5 (Finch and Ritchie, 1991). The whole section has been mega-brecciated by the deformation. It appears that most of the brecciation seen in Honduras is due to deformation rather than slumping of a platform margin. There is less evidence for early Cretaceous brecciation in Honduras than seen in Guerrero, Mexico. Most of the detrital limestones in Honduras are Ilama conglomerates of the upper Cretaceous Valle de Angeles group, after Mills and Barton (1996). The Ilama, originally described by Mills (1967), and later by Southernwood (1986), is composed of rounded limestone and volcanic clasts supported by a red to brown sandstone matrix. The conglomerates are probably due to block faulting of the Yojoa group carbonates before Chortis detached from southern Mexico. Widespread brecciation seen in Guerrero occurred later in Paleogene time during the detachment. Mylonitization on the Cacaguapa surface Manton and Manton (1989) describe a mylonitized shear zone on the Rio Cangrejal south of La Ceiba on the Caribbean coast that may be related to the La Ceiba fault system, which in turn may be an extension of the Jocotan fault zone on the Guatemala± Honduras border. For an excellent photograph of the Rio Cangrejal mylonite zone see the frontispiece of Tectonics 9(2), 1990.
Simonson (1977) describes a 202 kilometer exposure of granitic and tonalitic mylonite near El Provenir in the department of Francisco Morazan. The author speculates this zone may be related to the fault zone near San Francisco de la Paz (Fig. 5). Surprisingly there are few recorded exposures of mylonite along the suture zone between the Chortis block and the Maya block. Ophiolitic sequences, however, are common in this area, Finch and Dengo (1990). TECTONIC SUMMARY The tectonic events of the two regions are determined from previously published work as noted, and speculation by the author that is partially supported by the carbonate brecciation and distribution of mylonite zones. Late Cretaceous (Figs 7 and 10) The thick Early Cretaceous carbonate platforms were disrupted by an extensive magmatic arc between 104 and 65 Ma that extended along the Paci®c coast from Puerto Vallarta to the Acapulco area and then across northern Chortis where there was extensive block faulting. See Schaaf et al., (1995) and dates in Figs 4 and 5 that are shown next to small squares. Over 350 m of redbeds with rounded limestone conglomerates were deposited in block faulted valleys. Early Paleogene (Figs 7 and 10) The Cretaceous magmatism created a weak zone between the Acatlan complex and the Las Ovejas complex. A second period of intense igneous activity between 60 and 50 Ma along this weak zone formed
Fig. 5. Generalized tectonic map of northern Honduras. Heavy lines, major transcurrent faults; FZ, fault zone; small triangles, geologic stations; small squares, concordant radiometric dates in Ma. See Sedlock et al. (1993) and Manton (1987) for isotopes and methods.
Carbonate detritus and mylonite zones in Guerrero, Mexico 299
R. A. Mills Fig. 6. Measured section from photographs near San Francisco de la Paz, station 48. Section illustrates Valle de Angeles limestones and shales that have been severely deformed and mega-brecciated due to a granitic intrusion and related transtension faulting. Note locations of plates 4 and 5.
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the Xolapa and Cacaguapa complexes and at the same time severely ruptured and brecciated the platform carbonates, dumping them into several troughs, one of which was between Mazatlan and Tierra Colorada in Guerrero, and another one may have been along the Chamelecon fault zone in northern Honduras. Late Eocene±Early Oligocene (Figs 8 and 10) Schaaf et al. (1995) describe radiometric dates generally decreasing from 104 Ma near Puerto Vallarta to 21 Ma near Huatulco, a distance of 1100 km. This apparent migration of magmatism was due to changes of dip and velocity of the subducting Farallon plate. Hermann et al. (1994), also suggest that the Farallon subduction was oblique to the continent during the Paleogene. The Farallon subduction was probably related to the eastward advance of the Caribbean plate (Ross and Scotese, 1988). By 43 my, the vector of subduction (stage pole) was about 53 degrees azimuth (Engebretson et al., 1985). Therefore from Late Eocene to Early Oligocene time the subduction was to the northeast and it detached and carried eastward the various subblocks of Xolapa and Chortis. Magmatism also followed this eastward trend. Late Oligocene±Quaternary Figures 8 and 9 show the sub-blocks of the Xolapa and Chortis blocks being detached, riding over one another to some degree, and moving eastward. By Late Oligocene time the three Xolapa blocks were more or less in their present positions. The Chortis block, however, had left the Xolapa sub-blocks behind and was translating with the Caribbean plate. The actual route of the Chortis block is controversial. For example, with paleomagnetic data, Gose (1985) describes a 100 degree clockwise rotation and then a 100 degree counter-clockwise rotation before the northern margin of Chortis sutured to the Maya block in Late Oligocene or Early Miocene time. Field data presented in this paper suggests a post Cretaceous age for the break-up of Xolapa and Chortis; however, previously published radiometric data (Schaaf et al., 1995) narrow the time to between the late Eocene and the Late Oligocene, i.e., 43±24 Ma. During Neogene time, the vector of subduction was about 14 degrees azimuth (Pindell et al., 1988), or nearly perpendicular to the coast. This head-on subduction formed the younger plutons to the east around Huatulco. During Quaternary time, the subduction created the Acapulco and Middle America trenches with attendant volcanism responsible for continental accretion and the spectacular volcanic chain along the Paci®c coast of Central America (see Fig. 2 and Finch and Dengo 1990).
Carbonate detritus and mylonite zones in Guerrero, Mexico
Plate 4. A,B. Near San Francisco de la Paz, department of Olancho station 48. See Fig. 6 for location. Severely deformed and brecciated upper Cretaceous Valle de Angeles carbonates.
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Plate 5. A. Near San Francisco de la Paz, station 48. Complex section of lower Valle de Angeles carbonates showing altered breccia thrust over vertical beds of limestone and limestone breccia. B. Near San Francisco de la Paz, station 48. Highly weathered granite plug intruding lower Valle de Angeles brecciated carbonates.
Carbonate detritus and mylonite zones in Guerrero, Mexico
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Fig. 7. Paleotectonic mapÐlate Cretaceous±early Paleogene time. Projection is Lambert azimuthal equal area. See Fig. 1 for geographic localities. Small oblong areas are approximate locations of late Cretaceous igneous intrusions. Note vector of subduction for the Farallon plate.
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Fig. 8. Paleotectonic mapÐlate Eocene±early Oligocene time. Same projection as Fig. 7. See Fig. 1 for geographic localities. Note location for cross section C±C' that is represented on Fig. 10.
Carbonate detritus and mylonite zones in Guerrero, Mexico
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Fig. 9. Paleotectonic mapÐlate Oligocene time. Same projection as Fig. 8. NR, Nicaragua Rise; J, Jamaica. See Fig. 1 for geographic localities.
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Fig. 10. Diagrammatic sections along cross section C±C' shown on Fig. 8. There are six time sections for the Triassic, Jurassic, late Cretaceous, early Paleogene, late Eocene to early Oligocene, and late Oligocene. T/A, transcurrent fault; Tblock toward reader; A-block away from reader.
Chortis may have broken into sub-blocks that were oset several times by sinistral and dextral faults (Gordon and Muelberger 1994). Evidence for the subblocks includes previously described fault-related mylonite zones and the progressive easterly oset of the Honduras Caribbean coast along major fault zones (Figs 2 and 9). Although the Honduras fault zones are not well understood, it is possible they were oset by sinistral drag associated with the eastern translation of the Caribbean plate. The timing of the oset is not known. It may be the oldest faults are on the east side of Chortis while the younger ones are to the west.
CONCLUSIONS Field studies by the author in Guerrero and northern Honduras revealed similar zones of brecciation and mylonitization that are related to fault zones connected to the separation of Chortis from southern Mexico. It is suggested that the detachment of Chortis may have ruptured the Xolapa complex into three sub-blocks. These blocks rode over each other progressively to the east, leaving mylonitic scars. At the same time, the Cretaceous carbonate platform, covering the joint terrane, was severely deformed and col-
Carbonate detritus and mylonite zones in Guerrero, Mexico lapsed in southern Guerrero leaving thick accumulations of breccia. The absence of limestone and breccia south of the thick exposures, near Tierra Colorada, indicates that the Chortis block rotated over the Xolapa complex, leaving it barren. Chortis probably was broken into sub-blocks by sinistral forces related to the rotation of the Caribbean plate. Detrital limestone on Chortis is largely molasse type conglomerates ®lling valleys created by extensive block faulting in late Cretaceous time. Acknowledgements Ð The author is indebted to Max Suter and Richard Finch for original inspiration to start the study; Mark Gordon, who supplied many valuable references; Luis SanchezBarreda, who provided valuable contacts in Acapulco; Dante Moran-Zenteno, who personally, and with his sta, provided abundant data, suggestions and encouragement; and ®nally to my son, John, and my wife, Nohemi, who spent long days assisting with the ®eld work and preparation of the manuscript.
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