No abstract

The structural evolution that accompanied the breakup of Pangea during Jurassic time has been constrained in Mexico only at the regional scale on the basis of global plate tectonics and geometric considerations. According to available regional-scale reconstructions, the Jurassic tectonic evolution of Mexico was characterized by: (1) anticlockwise rotation of the Yucatán block along NNW-trending dextral faults and (2) sinistral block motions along W-to WNW-trending faults, which are geometrically needed to restore southern and central Mexico to the northwest of its present position during early Mesozoic time and avoid the overlap between North and South America in the reconstruction of Pangea. Reports of W-to WNW-trending sinistral faults that were active in Mexico during Jurassic time are presently few, and the existence, extension, and age of some of these structures have been questioned by many authors. In this work, we present the provenance analysis from a Jurassic clastic succession deposited within the Otlaltepec Basin in southern Mexico. Wholerock sandstone petrography integrated with chemical analysis of detritalgarnet and U-Pb detrital-zircon geochronology documents that the analyzed stratigraphic record was deposited during rapid exhumation of the Totoltepec pluton along the Matanza fault, which is a W-trending sinistral normal fault that extends along the southern boundary of the Otlaltepec Basin. U-Pb zircon ages and biostratigraphic data bracket the age of the Matanza fault between 163.5 ± 1 and 167.5 ± 4 Ma. This indicates that the Matanza fault was involved in the crustal attenuation that accompanied the breakup of Pangea and that sinistral motion of continental blocks along W-trending structures was taking place in southern Mexico as predicted by global plate tectonic reconstructions.

Mexico is an attractive place for provenance studies focused on reconstructing the tectonic evolution of North America. This is because Mexico hosts a well-preserved clastic record associated with some of the major Mesozoic and Cenozoic tectonic processes that shaped the face of this continent. However, the available information on Mexican pre-Mesozoic source terranes is presently insufficient for provenance analysis. With the aim of drawing the guidelines for provenance determination, we present here detrital modes, geochemical data, and zircon U-Pb ages for detritus derived from pre-Jurassic basement complexes of Mexico. Our data show that the various basement complexes produce distinctive detrital modes and supply diagnostic and compositionally different detrital heavy minerals that represent powerful provenance tracers. The Oaxacan Complex, Ayú Complex, and East Mexico Arc are the main sources of quartzo-feldspathic and feldspatho-quartzose detritus. Quartz with rutile needles, mesoperthitic K-feldspar, orthopyroxene, augitic to diopsidic clinopyroxene, and Mg- to Ca-rich almandine (Alm71–52Grs7–3Prp43–23Sps3–1Alm74–56Grs21–19Prp23–2Sps5–2) are common minerals in detritus from the Oaxacan Complex. The Ayú Complex supplies detritus marked by the occurrence of sagenitic biotite and white mica, as well as Mn-rich almandine (Alm69–66Grs4–3Prp18–11Sps19–10). Detritus from the East Mexico Arc contains any of these mineral phases ubiquitous in the Oaxacan and Ayú complexes. The Acatlán Complex is the main source of detritus dominated by metamorphic lithic grains and quartz, with minor amounts of feldspar. Lithic grains are rank 2–4 metabasitic, metapelitic, and metapsammitic–metafelsitic fragments. Diagnostic mineral phases are schorl–dravitic tourmaline, Na-amphibole, and helycitic garnet varying from a Ca- to Mn-rich almandine (Alm74–55Grs34–15Prp16–3Sps12–1-Alm70–46Grs20–15Prp3–1Sps32–12). Zircon U-Pb geochronology proves to have some virtues but also major limitations because: 1) the zircon U-Pb age signature of many different sources in Mexico is similar and 2) zircon documents a limited number of sources because of variations in zircon fertility.

During Pangea breakup, several Jurassic extensional to transtensional basins were developed all around the world. The boundaries of these basins are major structures that accommodated continental extension during Jurassic time. Therefore, reconstructing the geometry of Jurassic basins is a key factor in identifying the major faults that produced continental attenuation during Pangea breakup. We reconstruct the tectono-sedimentary evolution of the Jurassic Tlaxiaco Basin in southern Mexico using sedimentologic, petrographic, and U-Pb geochronologic data. We show that the northern boundary of the Tlaxiaco Basin was an area of high relief composed of the Paleozoic Acatlán Complex, which was drained to the south by a set of alluvial fans. The WNW-trending Salado River–Axutla fault is exposed directly to the north of the northernmost fan exposures, and it is interpreted as the Jurassic structure that controlled the tectono-sedimentary evolution of the Tlaxiaco Basin at its northern boundary. The eastern boundary is represented by a topographic high composed of the Proterozoic Oaxacan Complex, which was exhumed along the NNW-trending Caltepec fault and was drained to the west by a major meandering river called the Tlaxiaco River. Data presented in this work suggest that continental extension during Pangea breakup was accommodated in Mexico not only by NNW-trending faults associated with the development of the Tamaulipas–Chiapas transform and the opening of the Gulf of Mexico, but also by WNW-trending structures. Our work offers a new perspective for future studies that aim to reconstruct the breakup evolution of western equatorial Pangea.

The breakup of Pangea fundamentally shaped modern continents and controlled the climatic distribution and biodiversity on Earth. Although our knowledge on the processes that control the assembly and breakup of supercontinents has significantly improved, the timing of supercontinent assembly and breakup initiation remains in some cases controversial. According to the available data, the assembly of Pangea was completed in middle Permian time, resulting in the formation of major orogenic belts and transform faults like the N-S-trending, dextral, Caltepec fault in Mexico. Pangea breakup initiation is bracketed to early Middle Triassic time. In this work, we present new sedimentological, structural and U-Pb geochronological data from a fluvial unit, the Matzitzi Formation of southern Mexico, the age and tectonic setting of which have remained an enigma for the past century. Our data document that the Matzitzi Formation is the strati-EAGE MARTINI et al.