The reasons for, and mechanisms of, continental margin truncation in SW Mexico where Mesozoic-Cenozoic plutons are situated directly on the Pacific coast, are not yet well understood. Large-scale dextral and/or sinistral displacements of the continental margin terranes, now forming parts of Baja California or the Chortis block, have been proposed. The well-defined along-coast NW-SE decreasing granitoid intrusion age trend (-q.2 cm/yr in the 100 Ma-40 Ma time interval) between Puerto Vallarta and Zihuatanejo is interpreted by us to be a geometric artifact of oblique continental margin truncation rather than the consequence of a sinistral offset of the Chortis block from those latitudes toward the SE. Changes in the dip and velocity of the NNW-SSE trending Cretaceous-Tertiary subduction zone resulted in a landward migration of the magmatic arc. Taking into account certain stratigraphic affinities of Chortis and the Oaxaca and Mixteca terranes, together with the known displacement rates along the North America-Caribbean Plate boundary, the northwesternmost paleoposition of the Chortis block with respect to SW Mexico was near Zihuatanejo. In contrast, between Zihuatanejo and the Isthmus of Tehuantepec, the cessation of the Tertiary magmatism decreased more rapidly (-7.7 cm/yr), although the trend is not so obvious. Starting in the late Eocene, Chortis moved about 1100 km to the SE along a transform boundary associated with the opening of the Cayman Trough. Based on our geochronological data and structural relationships between mylonite zones and plutons in the Acapulco-Tehuantepec area, we propose an approximately 650 km SE movement of Chortis from about 40-25 Ma, with a velocity of 6.5-4.3 cm/yr. Since this is considerably slower than the decreasing age trend obtained by us using the geochronological data, we consider batholith formation in this segment to predate and postdate the offshore passage of the North America-Farallon-Caribbean triple junction. Geological observations and paleomagnetic data do not give strong support for large-scale right-lateral displacements of crustal blocks like the Baja California. Given the isotopic data presented, the 1Now at Instituto de Geologia, Universidad Naeional Aut6noma de M6xieo, Mexico City. Paper number 95TC01928. 0278-7407/95/95 TC-01928510.00 continental margin truncation in SW Mexico seems to be the consequence of an interaction of mechanisms. Of these, we regard tectonic erosion associated with the subduction process to be the most important in the northwestern segment. On the other hand, the lateral removal of material associated with the displacement of Chortis is more important in the southeastern segment. The tectonic evolution of the SW Mexican Pacific coast has been linked to the development of the Caribbean Plate and the displacement of the Chortis block to its present position [Malfait and Dinkelman, 1972; Pindell and Dewey, 1982; Anderson and Schmidt, 1983], including southeastern Guatemala, all of Honduras, E1 Salvador, and probably most of Nicaragua [Donnell...

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Elemental concentrations measured in 482 street dust samples from different urban and conurbated areas of Mexico City obtained using a portable energy-dispersive X-ray fluorescence spectrometer are reported. The data quality of eight different elements commonly used in pollution studies is evaluated, calculating the data precision and accuracy obtained with this handheld equipment. Precision for the various elements analyzed ranges between 1.2 and 15.9%, which can be considered as adequately precise for portable equipment. In the case of the accuracy, a maximum under-/overestimation of less than 20% was achieved. Pb and Zn correlation factors were 0.725 and 0.923, respectively. Similar mean Pb concentration values to those reported by ICP-OES were obtained. An average value of ~ 100 mg/kg for Pb concentration was estimated, which is significantly lower than the utmost permissible levels for residential/commercial usage. On the contrary, contamination factors calculated to identify the metal accumulation against those of the background values point from considerable to high contamination for 62 and 30% of the street dust samples analyzed, respectively. A comparison of the results recorded in Mexico City 10 years ago against those obtained suggests that no significant reduction in Pb concentration has occurred during this period despite the measures implemented to stop using leaded gasoline for three decades. Finally, the sample's low-amount requirements, simple nondestructive preparation procedure, and rapid elemental concentration determinations make portable XRF a promising geochemical determination technique for the fast and economic evaluation of contamination by heavy metals in large cities.

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