Chortis Block Research Articles

Geology of the coastal Chiapas (Mexico) Miocene plutons and the Tonalá shear zone: Syntectonic emplacement and rapid exhumation during sinistral transpression

Late Miocene plutons in coastal Chiapas, Mexico, represent the roots of an extinct magmatic arc. Miocene granitoids of calc-alkaline composition and arc chemistry intruded into and were deformed within the Tonala mylonite belt in the middle to upper crust. The mylonite belt is a crustal-scale shear zone extending along the western margin of the Chiapas Massif for ∼150 km. Deformation is characterized by a dominantly subhorizontal lineation and subvertical foliation along a strikingly linear zone that trends ∼310°. Mylonitic fabrics contain ambiguous but dominantly sinistral shear indicators. Intrusions are interpreted as syntectonic on the basis of similar U-Pb zircon crystallization age estimates (ca. 10 Ma) and the cooling age estimates obtained on neoformed micas in the mylonite. The plutons are elongated, their long axis is parallel to shear zone, and some plutons show markedly asymmetric outcrop patterns, with sheared tails that trail behind the intrusions and that are consistent with sinistral displacement. Parts of plutons were mylonitized by continuous deformation in the Tonala shear zone, locally developing intricate pseudotachylyte and cataclasite veins slightly oblique to the mylonite foliation. Outside of the shear zone, plutons preserve magmatic fabrics. These observations are consistent with features common to syntectonic granites interpreted to have been emplaced along strike-slip shear zones in a transpressional setting. We interpret the Tonala mylonites as representing a relict transform boundary that was slightly oblique to the Polochic-Motagua fault system, which accommodated over 100 km of sinistral displacement between the Chortis block (on the Caribbean plate) and Chiapas (on the North America plate) in late Miocene time.

Neotectonic development of the El Salvador Fault Zone and implications for deformation in the Central America Volcanic Arc: Insights from 4-D analog modeling experiments

The El Salvador Fault Zone (ESFZ) is an active, approximately 150 km long and 20 km wide, segmented, dextral strike-slip fault zone within the Central American Volcanic Arc striking N100°E. Although several studies have investigated the surface expression of the ESFZ, little is known about its structure at depth and its kinematic evolution. Structural field data and mapping suggest a phase of extension, at some stage during the evolution of the ESFZ. This phase would explain dip-slip movements on structures that are currently associated with the active, dominantly strike slip and that do not fit with the current tectonic regime. Field observations suggest trenchward migration of the arc. Such an extension and trenchward migration of the volcanic arc could be related to slab rollback of the Cocos plate beneath the Chortis Block during the Miocene/Pliocene. We carried out 4-D analog model experiments to test whether an early phase of extension is required to form the present-day fault pattern in the ESFZ. Our experiments suggest that a two-phase tectonic evolution best explains the ESFZ: an early pure extensional phase linked to a segmented volcanic arc is necessary to form the main structures. This extensional phase is followed by a strike-slip dominated regime, which results in intersegment areas with local transtension and segments with almost pure strike-slip motion. The results of our experiments combined with field data along the Central American Volcanic Arc indicate that the slab rollback intensity beneath the Chortis Block is greater in Nicaragua and decreases westward to Guatemala.

Origin and provenance of basement metasedimentary rocks from the Xolapa Complex: New constraints on the Chortis–southern Mexico connection

The U–Pb (LA-MC-ICPMS) geochronology of Xolapa metasedimentary rocks from Tierra Colorada, Guerrero to Puerto Ángel, Oaxaca in southern Mexico reveals that their protoliths accumulated in two distinctive cycles of sedimentation, one of Early Jurassic age and another of Late Cretaceous age. These ages are younger than thought and demonstrate that Xolapa metasedimentary rocks are not rocks from the Acatlán or Oaxacan complexes or their Paleozoic sedimentary covers as claimed. However, detrital zircon ages indicate that Xolapa sediments received contemporaneous detritus most likely from these assemblages suggesting a probably (para-)autochthonous origin for the Xolapa terrane. Xolapa rocks record two major tectonothermal events of 64–59Ma and ~34Ma; the first event produced the high-grade metamorphism and widespread migmatization that characterize Xolapa and the second event is likely related to extended heating produced by coeval arc plutonism.Pre-Jurassic assemblages of the Chortis block of Central America contain zircon populations that significantly coincide with those recorded in both, the Acatlán and Oaxacan complexes and their Paleozoic sedimentary covers as well as with those recorded in Xolapa metasediments, which suggests a spatial connection among these petrotectonic assemblages during much of the Mesozoic.It is proposed that Xolapa was generated in a basin floored by Permian rocks flanked on one side by southern Mexico terranes and on the other side by the Chortis block. Contraction of the basin tied to the approach and accretion of the Guerrero terrane arc assemblages during Late Cretaceous time produced crustal thickening and high-grade metamorphism and migmatization at mid-crustal levels. Diachronic exhumation of Xolapa began during Early Paleogene time very likely promoted by the detachment and migration of the Chortis block. The migration would additionally produce slicing of Xolapa assemblages generating its elongated and juxtaposed structure and the margin truncation that characterizes southern Mexico.

Mesozoic Oceanic Terranes of Southern Central America-Geology, Geochesmistry and Geodynamics.

The Northwestern edge of the modem Caribbean Plate, located in central Middle America (S-Guatemala to N-Costa Rica), is characterized by a puzzle of oceanic and continental terranes that belonged originally to the Pacific facade of North America. South of the Motagua Fault Zone, the actual northern strike slip boundary of the Caribbean Plate, three continental slivers (Copan, Chortis s. str. and Patuca) are sandwiched between two complex suture zones that contain HP/LT mafic and ultramafic oceanic rocks: The Motagua Melanges to the North, extensively studied in the last ten years and the newly defined Mesquito Composite Oceanic Terrane (MCOT) to the South. No modern geological data were available for the oceanic terrane located in the southern part of the so called continental “Chortis Block”. Classically, the southern limit of this block with the Caribbean Large Igneous Province (CLIP) was placed at a hypothetical fault line connecting the main E-W fault in the Santa Elena Peninsula (N-Costa Rica) with the Hess Escarpment. However, our study in eastern Nicaragua and northwestern Costa Rica evidences an extensive assemblage of oceanic upper mantle and crustal rocks outcropping between the Chortis/Patuca continental blocks and the CLIP. They comprise collided and accreted exotic terranes of Pacific origin recording a polyphased tectonic history. We distinguish: 1The MCOT that comprises a Late Triassic to Early Cretaceous puzzle of oceanic crust and are-derived rocks set in a serpentinite matrix, and 2The Manzanillo and Nicoya Terranes that are made of Cretaceous plateau-like rocks associated with oceanic sediments older than the CLIP. This study has been focused on the rocks of the MCOT. The MCOT comprises the southern half of the former “Chortis Block” and is defined by 4 corner localities characterized by ultramafic and mafic oceanic rocks of Late Triassic, Jurassic and Early Cretaceous age: 1 The Siuna Serpentinite Melange (NE-Nicaragua), 2The El Castillo Melange (Nicaragua-Costa Rica border), 3DSDP Legs 67 and 84 (Guatemala fore-are basin), and 4The Santa Elena Peridiotite (NWCosta Rica). The Siuna Serpentinite Melange (SSM) is a HP/LT subduction zone melange set in a serpentinite matrix that contains Oceanie crust and arcrelated greenschist to blueschist/eclogite facies metamafic and metasedimentary blocks. Middle Jurassic (Rajocian-Bathonian) radiolarites are found in original sedimentary contact with arc-derived greenstones. Late Jurassic black detrital chert possibly formed in a marginal (fore-arc?) basin shortly before subduction. A phregnite 40Ar/39Ar -cooling age dates the exhumation of the high pressure rocks as 139 Ma. The El Castillo Melange Mesozoic oceANic TeRRANes oF soUTHeRN ceNTRAL AMeRicA-GeoLoGY, GeocHesMisTRY AND GeoDYNAMics

The Padre Miguel Ignimbrite Suite, central Honduras: Paleomagnetism, geochronology, and tectonic implications

The Padre Miguel Group in western Honduras is a silicic volcanic sequence that forms part of the Central American Miocene volcanic arc built on the Chortis continental fragment. We report new 40Ar/39Ar data of 16.1±0.2Ma and 14.42±0.08Ma for the Padre Miguel Group, and present paleomagnetic data for 36 cooling units (mainly ignimbrites) from localities north and northeast of the capital city of Tegucigalpa. These rocks are characterized by univectorial characteristic magnetization carried mainly by low-Ti titanomagnetites, or two component magnetizations with a minor secondary overprint. Dual polarity magnetizations suggest that it is a primary thermo-remanence and the obtained data scatter can be straightforwardly explained by paleosecular variation. The overall mean of 33 selected sites (14.39°N, 87.14°W) defines a paleomagnetic pole at lat=80.0°N, lon=142.9°E (K=24.5, A95=5.2°). The overall mean compared with the North America expected direction indicates statistically insignificant rotation and inclination anomaly (R=−4.1°±6.1° and an F=−8.6°±11.3°). Restoring 255km of sinistral slip in the Polochic–Motagua fault system, as suggested by Cayman Trough reconstructions since 15Ma, brings the mid-Miocene arcs of southern Mexico (Oaxaca) and the Central America ignimbrite province into alignment; this is consistent with a derivation of the Chortis Block from southern Mexico. Our paleomagnetic and geochronological studies hence support models that explain the offset of the Miocene ignimbrite belt of Central America by post-Middle Miocene displacement of the Chortis Block relative to North America.

The volcano-tectonic evolution of the Miocene Santa Lucía Volcano, Boaco district, Nicaragua

*Corresponding author The present-day Santa Lucia caldera is an erosional relic of a Late Oligocene to Early Miocene stratoshield volcano located in the south-western part of the Chortis Block in Central Nicaragua. Six main lithological units were recognized: (Unit I) dacitic ignimbrite of Boaco type, which represents the basement of the Santa Lucia caldera; (Unit II) dacitic ignimbrite of Fonseca type, locally intercalated with epiclastic and dacitic lavas; (Unit III) “lower” andesite lavas; (Unit IV) blocky, lithic-rich pyroclastic flow deposits, (Unit V) “upper” andesite and basalt lavas, and (Unit VI) epic lastic rocks (lahar deposits). On the basis of field mapping, petrological and geochemical data, a new model for the evolution of the Santa Lucia Volcano is presented. The first stage consisted of a series of strong Sub-Plinian eruptions, which produced thick ignimbri te units. These events destroyed the pre-existing volcanic edifice. The second stage was dominated by large explosive eruptions producing mainly non-welded dacitic–andesitic ignimbrites. The next resulted in the formation of andesitic lava flows and minor tephra fall-out deposits, covered by voluminous basaltic lavas. Lahars probably triggered by volcanic and/or seismic events represent the final stage of volcanic activity. From the Miocene onwards, volcanic rocks became deeply weathered and locally eroded. Strongly weathered rocks are susceptible to landslides, and surficial modification by post-volcanic slope movements is observed on the slopes of the Santa Lucia Volcano. Identification of the lithological units and their comparison with wider Tertiary volcanic areas in Central Nicaragua will contribute to the knowledge of the regional volcanic stratigraphy and evolution.

Derivation of the Chortis and Chiapas blocks from the western Gulf of Mexico in the latest Cretaceous–Cenozoic: the Pirate model

Currently, two basic models describe the genesis of the Caribbean Plate: (i) a Pacific model that derives the Caribbean Plate off southern Mexico and (ii) an in situ model. The Pacific model requires the 1100–1400 km sinistral displacement recorded across the Cayman Trough to pass through the Gulf of Tehuantepec into the Middle America Trench, but no evidence of such a connection exists. The in situ model is inconsistent with the 1100–1400 km displacement across the Cayman Trough. A way through this impasse is indicated by the northwestward curvature of active oblique reverse to sinistral transcurrent faulting in southeast Mexico. Extending this potential solution back to ca. 80 Ma forms the basis of the new Pirate model, in which the Caribbean Plate and the Chortis and Chiapas blocks are derived from the northwest by anticlockwise rotation during the latest Cretaceous and Cenozoic. Following passage of the Chortis Block, the northern and southern parts of the Yucatan block collided along the intra-Yucatan suture, producing the 11–9 Ma Chiapas fold-and-thrust belt. The Pirate model accounts for the N-trending segment of the Laramide Sierra Madre Oriental–Zongolica foldbelts by anticlockwise drag, Palaeogene palaeocanyons, the second, 66–40 Ma phase of rifting in the western Gulf of Mexico, and post-10 Ma extension in the Chortis Block (Chortis–Sula rift province). Impingement of the East Pacific Rise on the Middle America Trench led to modification of the Pirate model involving subduction erosion of the ∼200 km-wide, Eocene–Oligocene forearc at ca. 25 Ma, opening of the Gulf of California at ca. 6 Ma, and birth and ESE movement of the Southern Mexico block (<5 Ma) followed by its fragmentation. The Pirate mechanism indicates that the North American Plate is relatively weak and so tears and rotates into the trailing edge of the Caribbean Plate.

An alternative Pangea reconstruction for Middle America with the Chortis Block in the Gulf of Mexico: tectonic implications

Understanding Pangea breakup requires a robust reconstruction, and this article focuses on the Middle America sector of the supercontinent. Although most Pangean reconstructions locate the Yucatan Block along the southern USA, the Chortis Block is generally placed off southern Mexico (Pacific model), undergoing sinistral relative motion during the Mesozoic and Cenozoic. However, the Pacific model is inconsistent with the absence of a Cenozoic fault linking the Cayman transforms and the Middle America Trench. We present an alternative Pangean reconstruction, where both the Yucatan and Chortis Blocks are placed in the future Gulf of Mexico, moving Mexico westwards along the Mojave–Sonora megashear to accommodate overlap with South America. Subsequent Mesozoic and Cenozoic evolution is inferred to have occurred in two stages: (i) Jurassic clockwise rotation along the Mojave–Sonora and West Florida megashears, followed by (ii) Cenozoic anticlockwise rotation along the Sierra Madre Oriental and East Yucatan megashears. The first stage is linked to the breakup of Pangea where the Gulf of Mexico formed as a pull-apart basin. The second stage is related to the evolution of the Caribbean where the Chortis and Yucatan Blocks rotated into the trailing side of the Caribbean Plate (pirate model). The new reconstruction is consistent with major parameters, such as (i) gravity, magnetic, and palaeomagnetic data; (ii) the westward continuation of the Cayman transform faults through the Chiapas foldbelt and along the N–S front of the Sierra Madre Oriental foldbelt; (iii) the 27–19 Ma removal of the southern Mexican forearc; (iv) offset of the Cretaceous volcanic arc (Guerrero-Suina); (v) the deflection of the Laramide orogen (Sierra Madre Oriental–Zongolica–Colon); and (vi) the continuity of Cretaceous platformal carbonates containing Caribbean fauna across Middle America. In this latter context, the Motagua high-pressure belt is interpreted as a Cretaceous extrusion zone into the upper plate above a subduction zone rather than as an oceanic suture.

Oligocene‐Miocene back‐thrusting in southern Mexico linked to the rapid subduction erosion of a large forearc block

Both the timing and mechanism for the removal of a ∼150–250 km wide forearc block from southern Mexico during the Cenozoic are controversial. Principal competing hypotheses are (1) removal due to sinistral strike‐slip shear, in which slow, diachronous removal of the Chortis Block throughout the Cenozoic is inferred, and (2) removal due to subduction erosion, in which rapid removal of a large forearc block during the late Oligocene/early Miocene is inferred to be synchronous with the rapid landward migration of the southern Mexican arc. New data indicate northeast‐directed back‐thrusting in (1) the Chacalapa shear zone west of −96.5°E, with the timing of shear deformation bracketed by a 25.5 ± 0.5 Ma U/Pb zircon age and a 20.7 ± 0.6 Ma Ar/Ar biotite age, and (2) in an unnamed shear zone to the south, with the timing of deformation bracketed by a 27.5 ± 0.5 Ma U/Pb zircon age and a 25.1 ± 0.2 Ma Ar/Ar biotite age. Zircon and biotite ages date the emplacement and cooling of deformed plutons, respectively. The observed back‐thrusting is consistent with a model of forearc removal due to subduction‐erosion processes because it is evidence for subduction‐orthogonal shortening occurring within the upper plate just before the landward migration of the southern Mexican arc. Rapid subduction of the southern Mexican forearc could have recycled continental lithosphere into the upper mantle at a rate up to half the global average rate of subduction erosion during the late Oligocene/early Miocene.

The Chortis Block--southwestern Mexico connections: U-Pb zircon geochronology constraints

The proposed connections between basement terranes of southwestern México and the Chortís Block of Central America are tested in this paper, which focuses on metamorphic units in northern Chortís that outcrop in southeastern Guatemala. Two chief units are recognized: the mid- to high-metamorphic grade Las Ovejas Complex made up of ortho-, para-gneisses, schists, amphibolites and marbles; and the tectonically juxtaposed, low-metamorphic grade San Diego Phyllite. U-Pb geochronology carried out by LA-ICPMS on zircons separated from both units reveal major differences between them. The high-grade metamorphism and deformation of Las Ovejas Complex is bracketed between ∼27 and ∼37 Ma, which correspond to the U-Pb zircon ages of the oldest cross-cutting dikes and youngest zircons in metasedimentary samples. Las Ovejas samples share xenocrystic and detrital zircon populations of Middle-Late Jurassic, Middle-Late Triassic and Permo-Carboniferous age. In contrast, the San Diego Phyllite contains zircons that overlap at ∼830 to 870 and ∼1150 to 1220 Ma, with the youngest concordant zircons yielding Cambrian age. The contrast in population ages supports the idea that Las Ovejas Complex and San Diego Phyllite are different lithotectonic units tectonically juxtaposed during or after their exhumation. When compared with available U-Pb zircon data for southern México, Las Ovejas Complex show similarities to samples of the Guerrero terrane, mainly the Arteaga Complex and Zihuatanejo subterrane, as well as to the southern and northern portions of the Cuicateco terrane. The San Diego Phyllite shows instead similarities with the Teloloapan subterrane of the Guerrero terrane, the Mexcala Formation, the low-grade units of the Cuicateco terrane and the Grenvillian ages of the Oaxacan Complex metaigneous samples, as well as some of the samples belonging to the Maya Block. We propose a tectonic model in which the Las Ovejas Complex was a portion of a fringing arc located in front of the Cuicateco terrane of southwestern México, removed during the Early Tertiary by docking of the Chortís Block represented by the San Diego Phyllite, and tentatively by similar units recognized elsewhere in the Chortís Block in Honduras and Nicaragua. Eocene-Oligocene metamorphism and deformation of the Las Ovejas Complex would be a result of its displacement off southern México, mainly characterized by sinistral stretching and tectonic Cenozoic transport.

Along and across arc geochemical variations in NW Central America: Evidence for involvement of lithospheric pyroxenite

The Central American Volcanic Arc (CAVA) has been the subject of intensive research over the past few years, leading to a variety of distinct models for the origin of CAVA lavas with various source components. We present a new model for the NW Central American Volcanic Arc based on a comprehensive new geochemical data set (major and trace element and Sr–Nd–Pb–Hf–O isotope ratios) of mafic volcanic front (VF), behind the volcanic front (BVF) and back-arc (BA) lava and tephra samples from NW Nicaragua, Honduras, El Salvador and Guatemala. Additionally we present data on subducting Cocos Plate sediments (from DSDP Leg 67 Sites 495 and 499) and igneous oceanic crust (from DSDP Leg 67 Site 495), and Guatemalan (Chortis Block) granitic and metamorphic continental basement. We observe systematic variations in trace element and isotopic compositions both along and across the arc. The data require at least three different endmembers for the volcanism in NW Central America. (1) The NW Nicaragua VF lavas require an endmember with very high Ba/(La, Th) and U/Th, relatively radiogenic Sr, Nd and Hf but unradiogenic Pb and low δ18O, reflecting a largely serpentinite-derived fluid/hydrous melt flux from the subducting slab into a depleted N-MORB type of mantle wedge. (2) The Guatemala VF and BVF mafic lavas require an enriched endmember with low Ba/(La, Th), U/Th, high δ18O and radiogenic Sr and Pb but unradiogenic Nd and Hf isotope ratios. Correlations of Hf with both Nd and Pb isotopic compositions are not consistent with this endmember being subducted sediments. Granitic samples from the Chiquimula Plutonic Complex in Guatemala have the appropriate isotopic composition to serve as this endmember, but the large amounts of assimilation required to explain the isotope data are not consistent with the basaltic compositions of the volcanic rocks. In addition, mixing regressions on Nd vs. Hf and the Sr and O isotope plots do not go through the data. Therefore, we propose that this endmember could represent pyroxenites in the lithosphere (mantle and possibly lower crust), derived from parental magmas for the plutonic rocks. (3) The Honduras and Caribbean BA lavas define an isotopically depleted endmember (with unradiogenic Sr but radiogenic Nd, Hf and Pb isotope ratios), having OIB-like major and trace element compositions (e.g. low Ba/(La, Th) and U/Th, high La/Yb). This endmember is possibly derived from melting of young, recycled oceanic crust in the asthenosphere upwelling in the back-arc. Mixing between these three endmember types of magmas can explain the observed systematic geochemical variations along and across the NW Central American Arc.

Crustal structure of Mexico and surrounding regions from seismic ambient noise tomography

SUMMARY Using continuous seismic data from newly available broadband stations in Mexico and Central America we have obtained group and phase velocity maps of fundamental mode Rayleigh wave for the region. These new maps have been calculated for periods between 8 and 60 s from cross-correlations of seismic ambient noise between 100 broadband stations, and stacked for 30 months from 2006 to 2008. The tomographic inversion of the obtained dispersion measurements has been carried out on a 1°× 1° grid, resulting in maps with resolution better than 250 km in the well-sampled regions of the model. For short periods (8–16 s) dispersion maps show good correlation with surface structural features. Low-velocity anomalies correlate with sedimentary basins around the Gulf of Mexico and Colorado embayment. High-velocity anomalies at short periods correlate with mountain ranges and regions of thin, extended crust such as the Gulf of California. Both the lowest and highest group velocity anomalies at these periods reach values of up to 15 per cent. For periods between 25 and 40 s, velocity anomalies are related to variations in crustal thickness and temperature. The most prominent low velocity anomaly correlates with thick crust and high mantle temperatures associated with the Trans-Mexican Volcanic Belt and the Central American volcanic arc. The most remarkable features for longer periods (40–60 s) are the differences between high-velocity anomalies beneath the Mayan block, the Gulf of Mexico and the cratonic part of the United States and low-velocity anomalies beneath the Chortis block and northern Mexico. The long wavelength features of our model agree well with previous global and continental scale studies. However, because of the increased station density of the data set used, we are able to obtain reliable dispersion maps for shorter periods, and to image smaller scale features.

The Chuacús Metamorphic Complex, central Guatemala: geochronological and geochemical constraints on its Paleozoic - Mesozoic evolution

The Chuacus Metamorphic Complex is located in Central Guatemala, between the Polochic and Motagua fault zones. It is made up of complexly intercalated, mafic and felsic high-grade gneisses, amphibolites, pelitic and quartzofeldsphatic metasediments and subordinate marbles. Mafic dikes and lenses metamorphosed to amphibolite and eclogite facies are tholeiitic and similar to mid-ocean ridge basalts. In contrast, metamorphosed intrusives (gabbro, diorite and granite) are calc-alkaline and have the geochemical signature of arc magmas. Laser Ablation Inductively Coupled Plasma Mass Spectrometry U-Pb zircon geochronology allows the recognition of three episodes of metamorphism. The first eclogite facies metamorphism (M1) is bracketed between Ordovician magmatism in the northern Chuacus Metamorphic Complex and the neighboring Rabinal granitic suite; the second corresponds to an Upper Triassic period of arc magmatism and migmatization (M2); the third high-grade metamorphic event (M3) occurred during the Late Cretaceous. The tectonic evolution of the Chuacus Metamorphic Complex began during the Early Paleozoic as a basin in the Rheic Ocean that received detrital material from the Maya Block, Acatlan and southeastern Mexico. The Chuacus Metamorphic Complex evolved to an active margin that subducted to HP conditions during the Mid- Late Paleozoic, and then was exhumed and involved in two tectonothermal events during the Upper Triassic and Late Cretaceous. The Chuacus Metamorphic Complex was accreted to the southern Maya Block during the Late Cretaceous, as a result of the convergent tectonics between the latter and either the Greater Antillean arc or the Chortis Block

Geochemistry and petrology of three granitoid rock cores from the Nicaraguan Rise, Caribbean Sea: implications for its composition, structure and tectonic evolution

The Nicaraguan Rise is a major submarine structure of poorly known origin. Its lithologies have been studied from dredge hauls and land outcrops on the Greater Antilles and Central America and its structure from geophysical data. In this paper we present the first geochemical analyses for granitoids that were recovered during the 1970s from cores drilled on the Nicaragua Rise for oil prospecting. The three Nicaraguan Rise rocks are calk-alkaline granitoids, and lie in the high-K field for Caribbean granitoids similar to the Above Rocks, Jamaica and Terre Neuve, Haiti intrusions. All of these intrusions are considered to be of Late Cretaceous – Paleocene age. Key elements abundances - K, La, Ce, Nd, Hf, Zr and Sm - indicate that the three Nicaraguan Rise rocks present more affinity with mature oceanic arc rocks similar to other granitoids from the Greater Antilles rather than mature continental arcs. The Pb, Nd and Sr isotope data show no evidence of a continental component, thus indicating that the more eastern and northern submarine area of the Northern Nicaraguan Rise is not underlain by continental crust of the Chortis block. Although of similar age, the Nicaraguan Rise samples are different from the more depleted Cuban granitoids of the Sierra Maestra, though both show strong similarities in their 207Pb/204Pb composition. We postulate that the Northern Nicaraguan Rise was most likely a Caribbean oceanic arc system that may have interacted only at its margin with the continental blocks bounding the region to the west in the area of the Northern Honduran borderland

Historia del Antearco Mesoamericano Austral y sus implicaciones para la evolución de la placa del Caribe

The cornplex, pre-Campanian history of northem Costa Rica results from allochthonous slabs, which are the remains of an old accretionary prism, which can be traced back to the subduction of proto-Caribbean crust during late Jurassic to Cretaceous times before the southerly Chortis block. The deactivation of this subduction system place in Cenomanian times. It was only when the subduction process taking place there was completed that an inter-American subduction zone was formed, from which the Central American island arc systems later emerged. A left lateral strike-slip-system later developed in the area of the former northern Caribbean subduction zone, represented today by the Hess Escarpment. Thrusts in a southerly direction took place along this transform margin during the Campanian. The remains of the former subduction complex were thus transported southwards, and thrusted to the primitive island arc rocks of northern Costa Rica. Owing to this structural evolution of the Central American are the origin of the thickened Caribbean crust ("Horizon B”) cannot be explained by the passive insertion of an pacific oceanic plateau. The Caribbean sill event is therefore interpreted to be a parautochthonous fomation in the western Caribbean and eastern part of the Phoenix plate. The basin systems of Central America demonstrate a similar, if not identical development, starting at the late Campanian. Five 2nd order depositional sequences can be identified within the foreaar basins, which can be correlated with the cycles ZC-4, TA l, TA 2, TA 3, and TA 4 (HAQ et al., 1988). Although thick submarine systems have been documented in the forearc basins of south-western Nicaragua and northem Costa Rica as far as the Eocene, the south-lying island are segments (southern Costa Rica, Panama) are mainly charcterized by hemipelagic sedimentation systems which, as a result of basaltic lava, breccias and chaotic mass flow deposits, are indicative of a fault-controlled enviroment in the area of a transform fault. This transform fault may have been the link to island arc segments lying further south which collided with the Central Cordillera of South America during late Cretaceous to early Tertiary times. The northely and southerly island are segments of Costa Rica and Panama only demonstrate a similar sedimentary development from the middle Eoceno, thereby signifying the formation of a continuous inter-American subduction zone. The internal architecture of the depositional sequences is mainly charcterized by the fomation of repetitive lowstand systems tracts. Either sandrich tabular lobe systems (Type I, cf. MUTTI &amp; NORMARK, 1987) or channellobe systems of smaller dimensions (Type II, cf., MUTTI &amp; NORMARK, 1987) developed, depending on the supply of sediment and tectonics. These in tum were overlain by thick channel-overbank complexes or prograding wedges (Type III, cf. MUTTI &amp; NORMARK, 1987). The plant material and resediment neritic fossil content is generally high in lowstand systems tract deposits. The composition of these varies in accordance with the position of the relative se level. The depositional systems of the transgressive and highstand systems tracts can only be segregated in areas of low sedimentation rates. In these areas, il typically consists of fine-grained calcareous deposits. The fast rise and highstand of the sea level is therefore documented through the progressive increase in the carbonate content.

The Greater Antillean Arc: Early Cretaceous origin and proposed relationship to Central American subduction mélanges: implications for models of Caribbean evolution

We integrate known and suspected Meso-American plate motions with the geochronology of Caribbean arcs and Central American HP/LT belts to promote two primary concepts. The first is that the Greater Antillean Arc formed at about 135 Ma by inception of SW-dipping subduction along a sinistral ‘inter-American transform’ connecting the west-facing subduction zones of the North and South American Cordillera. Caribbean arc magmas date from 135 Ma and Caribbean HP/LT complexes from 118 Ma, and they were formed as the Caribbean lithosphere was engulfed between North and South America during westward drift of – and divergence between – the latter. Initial arc magmatism predates exhumation of HP/LT metamorphic rocks by 10–20 million years; that is, magmatism developed faster than refrigeration and recycling in the subduction zones. However, pre-130 Ma HP/LT mélanges in Nicaragua (Siuna complex, 139 Ma 40Ar/39Ar) and in Guatemala both north and south of the Motagua fault zone (140–130 Ma Sm–Nd ages) require a different explanation. Therefore, our second concept is that these older rocks originated within a mature, west-facing Cordilleran subduction channel west of Mexico and/or the Chortis Block, and were subsequently dragged southeastward during sinistral subduction to a position along the NW portion of the ‘inter-American transform’. Upon initiation of SW-dipping subduction along this transform at 135 Ma, these older HP/LT (and minor arc) rocks became entrained within the transpressive zone between the western end of the Caribbean Arc and the southeast-facing Proto-Caribbean passive margin of the Chortis and Maya blocks. They were obliquely and diachronously accreted along these margins in the Albian to Maastrichtian. Parts of these mélanges are now exposed on both the northern and the southern flanks of the Motagua Valley of central Guatemala, and we further develop earlier suggestions that the southern Motagua mélanges were emplaced there on Chortis basement units by mid-Tertiary south-vergent transpression along the Motagua Fault.

Tectonics of southwestern Mexico, isotopic evidence, nuclear Central America, Late Cretaceous break up

In the paleogeographic reconstruction of Mexico and northern Central America, an ever-increasing amount of evidence shows that the entire region is a collage of suspect terranes transported from abroad, whose timing and sense of motion are now beginning to be understood. Among these, the Chortis block (nuclear Central America) and the Baja California Peninsula have been proposed as pieces of continent separated from the Pacific coast of southwestern Mexico, that have moved either southeastward by the Farallon plate or northwestward by the Kula plate. Previous studies mainly confined to the northern margin of the Chortis block, confirmed a left-lateral displacement of 130 km in Neogene time. Further studies made northwestward along the Mexican coast provided a better understanding of magmatic and metamorphic processes in the area, and suggested times of detachment increased to 30 Ma, 40 Ma, and 66 Ma. The pre-detachment westernmost position of the block has changed, depending on the model chosen, from Puerto Vallarta and beyond, to the current position. Here we show that the isotopic mineral ages from coastal granites along the coast from Puerto Vallarta, Jalisco (80 Ma) to Puerto Angel, Oaxaca (11 Ma) record systematic decrease of cooling ages from NW to SE. This pattern is interpreted to result from the progressive uplift of rocks exposed at the present-day coast in that direction, such uplift occurred in response to the development of the Middle America Trench at the newly formed continental margin when the Chortis block was sliding at an average rate of 1.5 cm/year in a sinistral sense to its present position. Our results also constrain the position of the Kula-Farallon spreading axis north of Puerto Vallarta. These observations led us to conclude that several indicators point to this time and region for the onset of strike-slip drifting of the Chortis block toward its current position. Here, we also present several view points in terms of other possilble interpretations to different tectonic, geologic and isotopic data sets published recently by different authors.

Age and Geochemistry of the Central American Forearc Basement (DSDP Leg 67 and 84): Insights into Mesozoic Arc Volcanism and Seamount Accretion on the Fringe of the Caribbean LIP

The igneous forearc basement along the Pacific coast of northern Central America (between southern Mexico and Costa Rica) comprises a highly tectonized accretionary assemblage of igneous and ultramafic rocks. Volcanic and gabbroic rocks with primitive arc geochemical signatures formed between ∼100 and ≥180 Ma and are interpreted to have originated by arc magmatism resulting from subduction of the Pacific–Farallon plate. Geochemically enriched ocean island basalt (OIB)-like units are interpreted as accreted seamounts and islands of a hotspot track, which was active between ≥220 and 100 Ma and originated from a hotspot located in the central Pacific. Based on their combined Pb, Nd and Hf isotopic compositions an affiliation of these rocks with the Caribbean Large Igneous Province or the present-day Galapagos hotspot appears unlikely. Rocks of similar age and geochemistry are exposed on the Santa Elena Peninsula in Costa Rica, suggesting that a similar forearc basement is accreted to the continental Chortis Block from southern Mexico to Costa Rica.

Middle-Late Ordovician magmatism and Late Cretaceous collision in the southern Maya block, Rabinal-Salama area, central Guatemala: Implications for North America-Caribbean plate tectonics

The Rabinal-Salama area in central Guatemala provides critical data bearing on the relationships between the North American and Caribbean plates because it lies within the Polochic-Motagua fault zone that separates the two plates. The cumulative Cenozoic sinistral displacement across this zone that separates the Maya and Chortis terranes has been variously estimated to be ~125 km or ~1100 km, evidence for which should be recorded in the rocks of the studied area. The Rabinal-Salama area lies between two of the east-west faults within the Motagua fault zone, the Polochic fault, and the Baja Verapaz shear zone. The shear zone separates the Maya block from eclogitic rocks of the Chuacus Complex that pass southward into ophiolitic rocks and melanges that define a suture between the Chuacus Complex and the Chortis block. The following sequence of events is recorded in the Rabinal-Salama area: (1) low-grade, pre-Silurian siliciclastic metasedi-mentary rocks (San Gabriel unit), that are intruded by (2) ca. 462–453 Ma calc-alkaline, peraluminous, S-type Rabinal granite suite, and unconformably overlain by (3) very low grade clastic and calcareous metasedi-mentary rocks (Santa Rosa Group) containing Mississippian conodonts and pebbles of granite, sandstone, and phyllite derived from the older units. The Rabinal granite suite is inferred to be rift related, inheriting its calc-alkaline signature from its source, along with the ca. 1 Ga xenocrystic zircons (upper intercept U-Pb data). Deformation in all these Paleozoic rocks produced a steeply south-southwest–dipping cleavage (chlorite and sericite) and a stretched quartz lineation. These fabrics become more intense adjacent to the Baja Verapaz shear zone, where C-S fabrics and rotated porphyroclasts indicate a reverse sense of motion with a sinistral component. White mica in the shear zone yields 74–65 Ma K-Ar ages, which are inferred to closely postdate the time of crystallization. Thus, although evidence for major sinistral displacement is absent, the kinematics are consistent with uplift and exhumation of the Chuacus Complex during obduction of the Baja Verapaz ophiolite onto the Paleozoic rocks of the Rabinal-Salama area in latest Mesozoic-Paleocene. This is inferred to have been produced during collision of the Cuban arc and Chortis block with the southern Maya block. Restoration of the Early Mesozoic ~70° anticlockwise rotation of the Maya block places the Rabinal-Salama area adjacent to northeastern Mexico, where comparable continental-shallow marine, Paleozoic rocks occur near Ciudad Victoria overlying the ca. 1 Ga Oaxaquia basement.

Constraints from finite element modeling on the active tectonics of northern Central America and the Middle America Trench

We have developed an elastic finite element model in order to study the role of the different forces acting on the northwestern part of the Central American Volcanic Arc and the Chortis Block. We present synthetic focal mechanisms, maps of tectonic regime, and strain crosses to analyze the results. The models show that to achieve the observed state of stress on the volcanic arc, the arc must be modeled as a lithospheric weak zone. Also, the forces related to the eastward drift of the Caribbean plate must be higher than those related to the subduction of the Cocos plate. The coupling on the subduction interface must be low, with or without slip‐partitioning due to the obliquity of the subduction at the trench. At Guatemala the western edge of the Chortis block is pinned against North America, even with low trench‐normal forces, making the triple junction between the Cocos, North American, and Caribbean plates a zone of diffuse deformation. The extension in the western part of the Chortis block, from Guatemala to the Honduras depression, is explained by the geometry of the North American‐Caribbean plate boundary and the direction of motion of the Caribbean plate with respect to North America. The direction of extension in the Chortis block is always E‐W regardless of the magnitude of the applied forces, and the main part of the deformation is absorbed between the Ipala graben and the Honduras depression, both features being consistent with our models.