U53A-01
Results From GPS Observations: No Evidence for Displacement Along the Polochic Fault in Southeastern Mexico
From 2000 to 2005 we performed GPS observations in nine sites (occupational) in southeastern Mexico. At the same time, french colleagues measured 24 sites in Guatemala. We obtained displacement vectors, with respect to a stable North America and with respect to a no-net-rotation frame of reference (ITRF2000). In Guatemala, where the trace of the fault has a well-defined surface trace, displacement vecors agree well with a left-lateral relative displacement along the fault. In southeastern Mexico, the trace of the fault is uncertain and the observed displacement vectors do not correspond to a left-lateral displacement, as predicted in earlier models. Our results show displacement vectors oriented in a NNE direction with respect to North America, and in a NW direction in the ITRF2000 reference frame. This suggests a clockwise rotation of the block located just to the northeast of where the fault trace ends. Some authors have suggested that such rotation takes place at the terminus of a strike-slip fault. Therefore, we suggest that Polochic fault actually ends where its surface trace dissapears, in the Mexico-Guatemala border.
U53A-02
Estimates of Cenozoic Forearc Subduction off Southwestern Mexico: Constraints on Eocene- Miocene Reconstructions
Traditional Cenozoic reconstructions of southern Mexico that place the Chortis block (Honduras and northern Nicaragua) adjacent southwestern Mexico using the Motagua Fault Zone violate plate tectonic principles, and are inconsistent with the presence of undeformed upper Cretaceous - Recent sediments in the Gulf of Tehuantepec that straddle the westward projection of the Motagua Fault Zone, the inferred northern boundary of the Chortis block. Rotating the Chortis block about a rotation pole near Santiago (Chile) places it to the southwest of its present position at 45 Ma with the Farallon Plate lying off southwestern Mexico. Examination of the geological record of southwestern Mexico (102-96°W) records several synchronous events in the upper Oligocene- lower Miocene (29-19 Ma): (1) an hiatus in arc magmatism; (2) removal of a wide (~210 km) upper Eocene-lower Oligocene forearc; (3) exhumation of 13-20 km of upper Eocene-lower Oligocene arc along the present day coast; and (4) breakup of the Farallon Plate. We suggest that the magmatic hiatus was caused by subduction of the forearc, which replaced the mantle wedge by relatively cool crust and has since been deeply subducted. Potential causes for subduction of the forearc include collision of an oceanic plateau with the trench, and a change in plate kinematics synchronous with breakup of the Farallon Plate and initiation of the Guadalupe-Nazca spreading ridge. Following removal of the forearc, arc magmatism resumed in two segments offset by the Veracruz-Oaxaca Line: (i) the 19 Ma to Present E-W Trans-Mexican Volcanic Belt; and (ii) the 22-10 Ma WNW part of the Central American Volcanic Arc. Estimates of the dip of the Benioff zone based on Miocene arc/forearc widths indicate that it changed from 11 to 39 degrees (west to east) across the Veracruz-Oaxaca Line, which is interpreted as a transform fault with a ~100 km dextral offset.
U53A-03
Ignimbrite Flare-up in the North-Central Sierra Madre del Sur, Southern Mexico: a Continuation of the Sierra Madre Occidental Ignimbrite Province?
The north-central Sierra Madre del Sur, immediately south of the Trans Mexican Volcanic Belt, is made up of a series of Eocene silicic volcanic centers distributed along a WNW trending 300 km long belt from Valle de Bravo to Huautla (98°50'-100°30'W longitude). This belt constitutes an inland arc near parallel to, and coeval with, the coastal batholitic belt of southern Mexico. It includes at least 9 exhumed volcanic edifices that display diverse caldera styles controlled by regional fracture zones and rheological contrasts in the host rocks. Initiation of the volcanism ranges from 37 Ma in the west to 32 Ma in the east, and extinction of magmatism was nearly simultaneous in the eastern half of the belt. Southwest of the silicic volcanic belt, andesitic volcanism is slightly younger (33-29 Ma) and the combined effect of post-Eocene uplift and erosion removed most of the extracaldera facies. The remnants of the silicic volcanic cover extend over an area of ~3000 km2. Reactivated fracture zones related to the volcanic activity include NW to W- trending faults that reveal a transition from left lateral to right lateral slip as result of a change in the stress field during the last stages of silicic volcanism. These and other groups of N-S to NNW trending faults accommodated the collapse structures and were the sites of emplacement of massive pyroclastic dikes. Immediately north of the Trans Mexican Volcanic Belt, ignimbrites and related rocks of the Mesa Central and southern Sierra Madre Occidental are slightly younger and display a westward 30 to ~15 Ma migration pattern accompanied by E-W regional extension. These patterns have been attributed to the roll back of the subducted slab, which caused the ascent of asthenospheric mantle and probably triggered ignimbrite flare-up. The distinct tectonic and geochronologic patterns of the arc volcanic rocks in the northern Sierra Madre del Sur and the southern Sierra Madre Occidental-Mesa Central suggests two slab segments with different geometry an dynamics during the early Oligocene that were decoupled at the latitude of the Trans Mexican Volcanic Belt.
U53A-04
Kinematics and age of Early Tertiary trench parallel volcano-tectonic lineaments in southern Mexico: Tectonic implications
We present new geological, structural, and geochronological data that constrain the timing and geometry of Early Tertiary strike slip deformation in southwestern Mexico and its relation with the concurrent magmatic activity. Geologic mapping in Guerrero and Michoacan States documented two regional WNW trending volcano-tectonic lineaments sub parallel to the present trench. The southernmost lineament runs for ~140 km from San Miguel Totolapan area (NW Guerrero) to Sanchiqueo (SE Michoacan), and passes through Ciudad Altamirano. Its southeastern part is marked by the alignment of at least eleven silicic to intermediate major domes as well as by the course of the Balsas River. The northwestern part of the lineament is characterized by ductile left lateral shear zones in Early Tertiary plutonic rocks observed in the Rio Chiquito valley. Domes near Ciudad Altamirano are unaffected by ductile shearing and yielded a ~42 Ma 40Ar/39Ar age, setting a minimum age for this deformation. The northern volcano-tectonic lineament runs for ~190 km between the areas of Huitzuco in northern Guerrero and the southern part of the Tzitzio fold in eastern Michoacan. The Huautla, Tilzapotla, Taxco, La Goleta and Nanchititla silicic centers (all in the range 37-34 Ma) are emplaced along this lineament, which continues to the WNW trough a mafic dike swarm exposed north of Tiquicheo (37-35 Ma) and the Purungueo subvolcanic body (~42 Ma). These rocks, unaffected by ductile shearing, give a minimum age of deformation similar to the southern Totolapan-Sanquicheo lineament. Post ~42 Ma deformation is essentially brittle and is characterized by several left lateral and right lateral transcurrent faults with typical Riedel patterns. Other trench-parallel left lateral shear zones active in pre-Oligocene times were recently reported in western Oaxaca. The recognizing of Early Tertiary trench-parallel and left-lateral ductile shearing in internal areas of southern Mexico suggest a field of widely distributed flow and shear zones with relatively small individual displacement that might represent an immature stage of the developing North American-Caribbean plate boundary. The documented transition from ductile to brittle deformation and the localization of shearing and volcanism in the Late Eocene may be related to the focusing of inter-plate deformation in a discrete left lateral transcurrent North America-Caribbean boundary. The opening of the Cayman Through at ~49 Ma may have accelerated this process.
U53A-05
Tectonic growth of a Cretaceous-Eocene accretionary orogen formed at the southern margin of the Caribbean Plate: integrated geological insights from northernmost Colombia
Geological characteristics from the Cretaceous to Eocene metamorphic and igneous basement rocks from the Guajira and the NW corner of the Sierra Nevada de Santa Marta massifs in the northern Colombian Caribbean region, and the stratigraphy of adjacent basins, reveal different stages of growth of a segmented Late Cretaceous to Eocene accretionary orogen formed by interaction between the NW margin of South America and the Caribbean plate. Extensive intercalations of metavolcano-sedimentary rocks in the Santa Marta region, chemical composition of spinels and pyroxenes from serpentinized mantle tectonites and gabbros, as well as whole rock geochemistry from basaltic dykes from the Guajira massifs, record the evolution of a Cretaceous intra-oceanic arc in a succesion of Mariana- and Chile-type subduction styles. Geological and positive gravity signatures suggest that this arc was accreted and tectonically underplated to the continental margin of South America, creating a Maastrichian-Paleocene syn-orogenic basin, presently cropping out in the Cesar-Rancheria valley. After this collisional event, subduction of the Caribbean plate under South America started, as revealed by intrusion of composite stitched Late Paleocene-Eocene granitoids in the NW corner of the SNSM and Guajira massifs. These plutons show continental calc-alkaline geochemical signatures and more evolved Sr and Nd isotopic compositions than juvenile and older intra-oceanic arc magmatic rocks. This new subduction environment is linked to the convergence between North and South America at this time. Low pressure estimations and fast cooling rates between 450-250° for the Eocene granitoids in the Guajira and Santa Marta regions, indicate shallow < 7 Km depths of intrusion. Local unconformable stratigraphic relationships with Eocene-Miocene sediments at the Guajira region and overimposed post-magmatic low temperature deformation in some of these granitoids suggest an important Late Eocene unroofing and deformational event that may record the regional modification of plate motion vectors and rates that controlled the Late Eocene- Miocene Caribbean orogenic phase.
U53A-06
An Updated Interpretation to the Position of the Kula-Farallon Ridge Against North America During the Late Cretaceous: Isotopic Evidence.
The initial break-up of the Pacific plate about 85 Ma ago into the Izanagi, Pacific, Kula and Farallon plates proposed by Woods and Davies [1982], has been documented by Engebretson et al., [1985] based on the absolute movement of the plates with respect to fixed Hot Spots (Hawaii and Yellowstone). Due to the lack of data from the oceanic crust that has already been subducted at the trench, in their corresponding reconstruction at 80 Ma ago, the uncertainty exists as to whether the crest was abutting against North America in southern Guatemala or north of Vancouver, an uncertainty of about 4500 km. Isotopic ages of crystalline rocks along the Pacific coast of Mexico from the state of Jalisco and the state of Oaxaca, a stripe 1,100 Km long, indicate a systematic decrease in Rb/Sr mineral ages, from 80 Ma in Jalisco to 11 Ma in Oaxaca, apparently related to the migration of the Chortis block (Honduras and Nicaragua) being detached from Mexico and transported by the Farallon plate toward the southeast. These results agree with previous published results. At Puerto Vallarta, Jalisco, the biotite mineral age obtained is 80 Ma, soon after the fragmentation of the Pacific plate proposed by Woods and Davies [1982]. This change in tectonic regime from a subduction zone to a transform margin is documented in that part of Mexico by a magmatic hiatus lasting roughly 25 Ma that initiates about 80 Ma ago, as well [Pantoja-Alor, 1983]. These data suggest that the Kula-Farallon ridge had to be located somewhere north of Puerto Vallarta, at the time of fragmentation of the Pacific plate 85 Ma ago. The counterclockwise paleomagnetic rotations reported from both the Pacific coast of Mexico and Honduras support these conclusions. On the other hand, the analyses of paleomagnetic data and field geology from British Columbia [Umhoefer, 1987] indicates a large displacement of that region along the Pacific margin of North America, of the order of 2,400 Km, from the present- day position of the Baja California peninsula to its current location, transported by the Kula plate to the north along a transform margin, between 85 Ma and66 Ma ago. This time interval is characterized as well by the absence of igneous activity in the Sierra Nevada. More recently [Umhoefer, 2003] has diminished the length of displacement of Baja to 1,650 Km assigning the position of the triple junction to a point in the neighborhood of the USA-Mexican border on the Pacific coast. From the apparently synchronous nature of the displacement of British Columbia to the north and Chortis to the south is inferred that the triple point Kula-Farallon-North America was located somewhere near the latitude of Puerto Vallarta at the time of break-up of the Pacific plate during the early Campanian, 85 Ma ago.
U53A-07
Piercing Lines Between Southwest Mexico and the Chortis Block of Northern Central America: Constraints on Cretaceous Position of Chortis Block
Geologic investigations in Honduras and Nicaragua reveal features and structures which correlate to southern Mexico in sufficient detail to constrain the latest Cretaceous position of the Chortis block along the truncated southwestern margin of Mexico. Two regional features common to southern Mexico and Honduras are 1) their Precambrian basement contain both containing Grenville age protoliths and 2) the similar Mesozoic cover of Late Cretaceous clastic, marine sandstone and shale over Early Cretaceous shallow water platform carbonate rocks. Three north trending piercing lines are common to southern Mexico and to the Chortis block: 1) north trending mid-Cretaceous arc and geochemical trends of Teloloapan arc in Mexico and the Manto arc in Honduras; 2) north trending late Cretaceous structural belts southern Mexico and the Frey Pedro, Comayagua, Minas de Oro and La Flor structural belts of Honduras and 3) a north trending common magnetic signature in Mexico separating the Guerrero terrane from the autochthonous terranes of Mexico and in Honduras forming the southwestern edge of the Precambrian continental basement. A forth piercing line is the east trending alignment of the eastern Honduras Colon fold belt with late Cretaceous east trending fold belt of southeastern Guatemala. Our best fit alignment of these features and piercing lines place the northern limit of the Chortis block along the southern margin of Mexico between Zihuatenejo and Acapulco in the latest Cretaceous and is consistent with 35 degrees of post-Cretaceous counterclockwise rotation of the Chortis block.
U53A-08
Protolith and metamorphic ages of gneiss hosting eclogite in the Chuacús complex, Central Guatemala
Epidote-amphibolite-facies gneisses in the Sierra de Chuacús, Central Guatemala, contain mafic domains with relics of eclogite-facies mineral parageneses. Field relations and petrography suggest that the protolith of orthogneiss was intruded by mafic dikes, subsequently subjected to eclogite-facies conditions, and finally strongly overprinted at epidote-amphibolite conditions. The host orthogneiss for an eclogitic band ca. 1m thick in the Agua Caliente River, and a quartz-pyroxene-garnet-zoisite gneiss from Los Altos were sampled for zircon SHRIMP-RG U/Pb dating and REE analyses. Cathodoluminescence images, Th/U, U/Pb and REE patterns of Agua Caliente gneiss zircon cores indicate magmatic crystallization between 217-229 Ma. Magmatic zircon cores of the Los Altos sample have similar texture, Th/U and REE, and yield 220-245 Ma ages. Coeval migmatite formation has been reported near Huehuetenango, Western Guatemala, suggesting extensive Triassic crustal melting and plutonism at the southern edge of the Maya block. A second population of Los Altos zircon cores show metamorphic textures with ages between 840-900Ma, suggesting a connection to Neoproterozoic orthogneisses in the northern flank of the Sierra de Chuacús. Bright CL metamorphic zircon rims from Chuacús gneiss contain low U (<60 ppm), and yield a weighted mean U238/Pb206 age of 75+/- 3Ma (n=7). Ages in the 217-80 Ma interval do not form clusters and are compatible with partial recrystallization of magmatic zircon or mixed analysis. Zircon rims of Los Altos gneiss show lower REE than magmatic cores, and steep heavy REE patterns. Zircon rims of Agua Caliente orthogneiss show less steep heavy REE patterns suggestive of growth in equilibrium with garnet, and are enriched in light REE. Eu anomalies in REE patterns indicate that cores and rims grew stable with plagioclase at crustal depth. This implies the ca.75 Ma metamorphic age represents the epidote-amphibolite overprint, and constrains the high-pressure event to have occurred after the Triassic and before the Late Cretaceous.