ABSTRACT The supercontinent of Pangea formed through the diachronous collision of Laurussia and Gondwana during the late Paleozoic. While magmatism associated with its formation is well documented in the Variscan orogeny of Europe and Alleghanian orogeny of the United States, little is known about the Sonora orogeny of northern Mexico. This paper reports geochronology (U-Pb zircon), whole-rock geochemistry, and Lu-Hf zircon isotope data on basement cores from the western Gulf of Mexico, which were used to develop a tectonomagmatic model for pre- to post-Pangea amalgamation. Our results suggest the existence of three distinct phases of magmatism, produced during different stages of continental assembly and disassembly. The first phase consists of Early Permian (294–274 Ma; n = 3) granitoids with geochemical signatures indicative of a continental arc tectonic setting. This phase formed on the margins of Gondwana during the closure of the Rheic Ocean, prior to the final amalgamation of Pangea. It likely represents a lateral analogue of late Carboniferous–Early Permian granitoids that intrude the Acatlán and Oaxacan Complexes. The second phase of magmatism includes Late Permian–Early Triassic (263–243 Ma; n = 13) granitoids with suprasubduction geochemical affinities. However, Lu-Hf isotope data indicate that these granitoids formed from crustal anatexis, with ε Hf values and two-step Hf depleted mantle model ages (T DM[Hf] ) comparable to the Oaxaquia continental crust into which they intrude. This phase of magmatism is likely related to coeval granitoids in the Oaxaca area and Chiapas Massif. We interpret it to reflect late- to postcollisional magmatism along the margin of Gondwana following the assembly of Pangea. Finally, the third phase of magmatism includes Early–Middle Jurassic (189–164 Ma; n = 2) mafic porphyries, which could be related to the synchronous suprasubduction magmatism associated with the Nazas arc. Overall, our results are consistent with Pangea assembly through diachronous collision of Laurussia and Gondwana during subduction of the Rheic Ocean. They suggest that postorogenic magmatism in the western termination of the Rheic suture occurred under the influence of a Panthalassan subduction zone, before opening of the Gulf of Mexico.

ABSTRACT New and existing strontium isotope data are given for several widespread evaporites from western equatorial Pangea. The data indicate evaporite deposition occurred on proximal margins of the Gulf of Mexico at ca. 169 Ma (Bajocian, not Callovian as commonly thought) and 166 Ma in Trinidad (Bathonian-Callovian boundary). The 166 Ma age may also apply to undated evaporite on the Bahamian margin, conjugate rift of Trinidad, and now in Cuba. We show that: (1) the Trinidadian (and Bahamian?) evaporite pertains to rifting rather than to Late Jurassic–Cretaceous carbonate platform deposition; (2) the Mata Espino-101B evaporite (a borehole in Veracruz Basin, Mexico) is not Paleocene but Bajocian (halite) or Bathonian (gypsum) and hence is not related to possible Paleogene Gulf of Mexico desiccation; (3) evaporite deposition may have offlapped basinward in the Gulf of Mexico (Bathonian–early Oxfordian in more distal areas), because most Atlantic opening models preclude the Gulf of Mexico from being large enough by 169 Ma to accommodate the mapped expanse of autochthonous salt deposition; and (4) a 3–9 m.y. hiatus (the Norphlet window) is apparent in proximal areas around the Gulf of Mexico between evaporite and upper Oxfordian marine successions, caused perhaps by proximal margin uplift (flexural or thermal) or by Gulf of Mexico water level remaining below paleo–sea level (evaporation?) during Bathonian–early Oxfordian time. Although a 20–30 m.y. hiatus may exist below evaporite in the U.S. coast, cordilleran Mexico was tectonically active into the Middle Jurassic, and pre-salt continental deposits are closer in age to salt deposition there. Pre-salt strata along Campeche–northern Yucatán remain undated. Our data do not resolve if the evaporite was sourced from the Atlantic, the Pacific, or both, but the fact that the Trinidadian evaporite is younger than Gulf of Mexico evaporite, and the presence of Bajocian marine and evaporite sections across Mexico perhaps favor the Pacific as the source.

As a result of the rapid advances in technology experienced during the last half of the twentieth century, methods used to study volcanoes have diversified enormously. Such diversification has created the illusion of a rapid advance in volcanology. Indeed, it is undeniable that the types of information and the amount of data available for studying diverse aspects of volcanoes have increased in the last decades, and are very likely to continue their growth in the near future. All of this information, however, does not seem to have solved many of the fundamental questions related to volcanic processes, not only in our planet, but in other parts of our solar system as well. Actually, the rapid development of new technologies and methods of measurement may have had a negative impact in the development of volcanology as a whole, because the super-specialization favored by the development of new technologies and methods has also limited communication among volcanologists with varying orientations. As a consequence, some of the fundamental problems in volcanology have been obscured by the quest for ever increasing accuracy of specific variables at the expense of deeper understanding of the general context. In this chapter we address this situation by identifying some of the challenging topics that, in our opinion, need to be explored in more detail by the community of volcanologists as a whole. Aspects such as the long-term evolution of composite volcanoes, volcano failure, and the internal structure of volcanoes are included in this list. An effort is made to formulate specific questions that can be solved in each of these sub-branches of volcanology, not by claiming the need for more specialized data, but rather by keeping an eye on the general context in which these problems occur. Thus, the topics examined in this work provide specific examples of how we envisage a solution for the biggest challenge of volcanology: fostering the communication among workers with different specializations with the goal of solving questions that are of a general nature.

Abstract We present an updated, internally consistent synthesis of the Permo-Triassic assembly and Mesozoic evolution of the Gulf of Mexico, Mexico, Florida–Bahamas and northern South America (Guiana margin and northern Andes), incorporating advances at regional, field and geochronological levels. The recently determined Bajocian age for salt deposition (using 87 Sr/ 86 Sr isotopes) is integrated by modifying the plate kinematic framework with a new Equatorial Atlantic reconstruction that expands the gap between the Americas by 180 km over many kinematic frameworks. NW–SE synrift lithospheric extension along western Florida–Bahamas is estimated at 40%, implying thinned continental crust beneath Great Bank, Bahamas, the conjugate for the Guianas Basin margin. In cordilleran Mexico (excluding the Yucatán Block), we propose two new means by which continental crust migrated into the ‘Colombian overlap position’ of Pangaean reconstructions. The first involved Jurassic–earliest Cretaceous sinistral displacement of the Oaxaca Block along a NW–SE ‘North Oaxaca Transfer’ through or adjacent to the Cuicateco Belt. The second applies to the continental crust in eastern Mexico to the north of Cuicateco, a region we refer to as ‘peninsular Mexico’. There, most Mesozoic basement faults trend NW–SE, and the common occurrence of Permian mid-crustal anatectic basement directly beneath Mesozoic red beds, salt and marine strata suggests extreme extension prior to the onset of sedimentation. Because these Mesozoic sedimentary sections typically sum to 3–8 km in thickness, the post-rift crust of peninsular Mexico probably averaged about 25 km in thickness before later orogenesis. Our reconstructions suggest that this Triassic–Middle Jurassic extension approached 100%, beginning with overthickened Alleghanian (Permian) crust about 50 km in thickness in palaeo-northern Mexico, and was accompanied by a significant sinistral component broadly distributed across the rift array. The updated model provides an exploration and kinematic framework for the entire region.