Abstract Since 1975 Mexican Mesozoic red-bed and salt sequences were dated using the palynostratigraphical method developed at the Mexican Petroleum Institute by Rueda-Gaxiola. They represent the beginning of the Mesozoic marine transgression, which eventually formed the petroleum systems of the region. The palynostratigraphical method is based on a macro- and microscopic analysis of all the components in palynological residues and of the glycerinated alcohol in which they are preserved. This analysis allows the identification of litho-, bio-, and stratigraphic data, which can be related to tectonosedimentary and diagenetic processes. The Cahuasas and the Rosario formations (in the Tampico-Misantla subbasin), the Todos Santos Formation (in the southeastern subbasins and in the western Veracruz subbasin), the La Boca Alloformation (in the Huayacocotla-El Alamar Basin), Consuelo Group (in the Tlaxiaco Basin), and some salt units (in the southeastern subbasins) have been dated ( J. Rueda-Gaxiola, 1972 ; J. Rueda-Gaxiola, 1976; L. Gutiérrez-Galicia, 1984 ; Herrera et al., 1990 ; J. Rueda-Gaxiola et al., 1993 – 1996 ; J. Rueda-Gaxiola; and J. Jiménez-Rentería, 2002). These data were integrated with those obtained by Kirkland and Gerhard from the cap rock of the Challenger Knoll in the center of the Gulf of Mexico. They have helped understand the sedimentary and tectonic evolutions of the early Gulf of Mexico. These data indicate the existence of an unconformity between the basement and the Middle Jurassic red beds (La Joya and Todos Santos formations) in the Sabinas and southeastern subbasins of the Gulf of Mexico. This unconformity is also found between the Liassic rocks (La Boca Alloformation and Huayacocotla Formation) and the Middle Jurassic red beds (La Joya and Cahuasas formations) in the Huayacocotla-El Alamar Basin. However, in the marginal Sabinas, Tampico-Misantla, and southwestern subbasins, this unconformity is overlain by Middle Jurassic red beds, which is slightly older than the evaporitic rocks deposited in the center of the Gulf of Mexico. However, some localities of the Tlaxiaco Basin and in the northern part of the Tampico-Misantla Basin, the Liassic Rosario Formation red beds gradually change to the Cualac and Cahuasas formations, which are the basis of the transgressive marine Upper Jurassic–Middle Cretaceous sequence.

Abstract The most important red-bed and salt sequences in Mexico are Jurassic and are located in eastern Mexico in or around the Gulf of Mexico. Most of these rocks are Middle Jurassic, and they are overlain almost always by evaporitic sequences that mark the beginning of the Middle Jurassic transgressive sequence. In some places, they overlie pre-Jurassic rocks. Mesozoic red-bed sequences have recently been dated with organic and inorganic components of palynological residues. Information from red-bed sequences in the Los San Pedros Allogroup and Huayacocotla Group (Rhaetic-Liassic age in the Huayacocotla–El Alamar Basin), La Joya Formation (Middle Jurassic age in the Sabinas Subbasin), Rosario and Cahuasas Formations (Middle Jurassic age in the Tampico-Misantla Sub-basin), and Todos Santos Formation (Middle Jurassic age in the Veracruz and Tabasco-Chiapas-Campeche Subbasins) allows us to construct a model for the origin and evolution of the Gulf of Mexico. The model includes three different stages: (1) the formation of one (or two?) Rhaetic-Early Liassic wrench or shear basin(s) (Huayacocotla–El Alamar Basin) related to the evolution of the Pacific convergent system; (2) formation of the Tampico-Misantla Sub-basin during the late Liassic as a result of the southwest displacement of the Huayacocotla and Tlaxiaco Blocks along the Tampico–Lázaro Cárdenas and Teziutlán-Acapulco Megashears; and (3), the origin during the Middle Jurassic of the Gulf of Mexico Basin and the Sabinas, Veracruz, and Tabasco-Chiapas-Campeche Mexican petroleum subbasins as a result of the development of a triple junction. This triple junction allowed the northwestward displacement of the Texas-Louisiana block and the western region of Mexico from the stable Chiapas-Tabasco-Campeche-Yucatán block along the Lewis Clark–Bahamas and Texas-Boquillas-Sabinas lineaments and the Pico de Orizaba–Laguna Inferior Megashear.

ABSTRACT The Oaxacan Complex is the largest outcrop of Grenville-age rocks in Mexico, constituting the main crustal fragment in the backbone of Oaxaquia. It is mainly composed of scarce metasediments, intruded by arc, alkalic, and tholeiitic magmas (ca. 1.3 to ca. 1.01 Ga) and later affected (ca. 0.99 Ga) by granulite-facies metamorphism. A detailed study, combining U-Pb geochronology by laser ablation–inductively coupled plasma–mass spectrometry, with in situ Hf isotopes in zircon grains, allowed comparison of the age and isotopic patterns of the Oaxacan Complex granulite rocks with those from other similar outcrops in Mexico (Huiznopala and Novillo Gneisses, Guichicovi Complex) and with the neighboring orogens such as the Grenville Province of the eastern United States and Canada, the Sveconorwegian orogen of SW Baltica, and some of the localities in which Mesoproterozoic rocks border the Amazonian craton of South America (Colombia, Peru, Brazil). Detrital zircon ages show that most metasedimentary rocks are younger than 1.4 Ga (only three samples contained zircon grains between 1.6 and 1.4 Ga), whereas U-Pb dating of igneous rocks (1245–1161 Ma) confirmed previous findings. Hf isotopes of dated zircon grains show that few crystals have negative ε Hf( t ) values, indicating a recycling component from an older crust, but most of them are moderately primitive, with ε Hf( t ) values of up to +12, and linear arrays parallel to the 176 Lu/ 177 Hf average crustal evolution model. Those Hf values are indicative of partial assimilation of an older crustal component, with Hf model ages of ca. 1.65–1.50 Ga. Comparison of these data helps to constrain possible Mesoproterozoic conjugate margins of Oaxaquia and propose a paleogeographic model in which Oaxaquia acted as the leading edge of Amazonia, together with the Colombian terranes, and received sedimentary input from different sources such as the southern Sveconorwegian orogen, the U.S.–Canada eastern Grenville Province, and some of the Mesoproterozoic belts bordering the Amazon craton.

The Caltepec shear zone is a dextral transpressional tectonic boundary between the Oaxacan and Acatlán Complexes, which are crystalline basements of the Zapo-teco and Mixteco terranes in southern México, respectively. The terrane boundary (2–6 km wide) reveals protracted and polyphase tectonic activity from at least Early Permian to the present. The major tectonothermal event in the Caltepec fault zone was related to the oblique collision of the metamorphic complexes during the amalgamation of Pangea. An anatectic leucosome and the resulting syntectonic granite (Cozahuico Granite) in the fault zone yielded U-Pb zircon ages of 275.6 ± 1.0 Ma and 270.4 ± 2.6 Ma, respectively. The initial 87 Sr/ 86 Sr ratios (0.70435–0.70686) and Sm-Nd model ages (T DM ) (1.0–1.6 Ga) for the Cozahuico Granite and leucosome indicate a magmatic mixture that originated from melted Proterozoic crust and a component of depleted mantle. The Leonardian age of the cover (Matzitzi Formation) and a 40 Ar/ 39 Ar cooling age (muscovite) of 268.59 ± 1.27 Ma for mylonitic mica schist at the base of the thrust imply high cooling rates (∼180 °C/Ma) and uplift during the Permian. The adjacent sedimentological record indicates intense tectonic reactivation during Early Cretaceous, Paleogene, and Neogene along the long-lived Caltepec fault zone, alternating with periods of relative tectonic quiescence during Triassic, Jurassic, and Mid-Cretaceous times. The trend of the Caltepec fault zone parallel to the Oaxaca fault, 50 km to the east, is interpreted as part of a synchronous and dynamically coupled tectonic system that has been releasing tectonic stresses associated with the rupture of Pangea and the evolution of the Pacific margin of southern México from Jurassic to Holocene times.

Abstract Gold mineralization in the Veraguas gold belt of Panama is hosted by calc-alkaline volcanic rocks of mid-Miocene age. Volcanic rocks range in composition from basalt to rhyolite and are intruded by coeval dikes, plugs, and exogenous volcanic domes of the same composition. Pyroclastic aprons surround the domes and host gold ore deposits. The Remance mine, a vein and vein stockwork, has produced gold intermittently for over a century. Santa Rosa, a shallow epithermal stockwork and disseminated deposit, went into production in 1995. The geologic setting of these deposits draws attention to the exploration potential of volcanic dome fields and related shallow epithermal (hot spring) systems throughout Central America.