Synthesis and tectonic interpretation of the westernmost Paleozoic Variscan orogen in southern Mexico: From rifted Rheic margin to active Pacific margin

Abstract

Paleozoic rocks in southern Mexico occur in two terranes, Oaxaquia (Oaxacan Complex) and Mixteca (Acatlán Complex) that appear to record: (1) Ordovician rifting on the southern margin of the Rheic Ocean, (2) passive drifting with Amazonia during the Silurian, (3) Devonian-Permian subduction beneath southern Mexico producing an arc complex that was partially removed by subduction erosion, subjected to HP metamorphism and Mississippian extrusion into the upper plate, followed by reestablishment of a Permian arc. In the Oaxaquia terrane, the 920–1300 Ma basement is unconformably overlain by a ∼ 200 m uppermost Cambrian–lowest Ordovician shelf sequence containing Gondwanan fauna (Tiñu Formation), unconformably overlain by 650 m of shallow marine-continental Carboniferous sedimentary rocks containing a Midcontinent (USA) fauna. In the Mixteca terrane, the low-grade Paleozoic sequence is composed of: (a) a ?Cambrian–Ordovician clastic sequence intruded by ca. 480–440 Ma bimodal, rift-related igneous rocks; and (b) a latest Devonian-Permian shallow marine sequence (> 906 m) consisting of metapsammites, metapelites and tholeiitic mafic volcanic rocks. High pressure (HP) metamorphic rocks in the Mixteca terrane consists of: (i) a Cambro-Ordovician rift-shelf intruded by bimodal rift-related intrusions that are similar to the low-grade rocks; (ii) periarc ultramafic rocks, and (iii) arc and MORB rocks. The Ordovician granitoids contain concordant inherited zircons that range in age from ca. 900 to 1300 Ma, indicating a source in the Oaxacan Complex. Concordant ages of detrital zircons in both the low- and high-grade Cambro-Ordovician metasedimentary rocks indicate a provenance in local Ordovician plutons and/or ca. 1 Ga Oaxacan basement, and distal northwestern Gondwana sources with a unique source in the 900–750 Ma Goiás magmatic arc within the Brasiliano orogen. These data combined with the rift-related nature of the Cambro-Ordovician rocks are most consistent with an origin along the southern margin of the Rheic Ocean. Latest Devonian-Permian deposition was synchronous with Mississippian extrusion of the HP rocks into the upper plate during extensional deformation. The HP Cambro-Orodivician rift-shelf rocks are inferred to have originated in the forearc region of the upper plate that was removed by subduction erosion, carried down the subduction channel, and then extruded into the upper plate in the middle of the Mixteca terrane. The presence of arc-related rocks in the HP assemblage suggests that the arc complex was also removed, whereas the MORB rocks may have been derived from the subducting slab. Detrital zircons in the Carboniferous rocks of both the Mixteca and Oaxaquia terranes contain Devonian detrital zircons and volcanic clasts that are inferred to have come from the removed Devonian arc on the western margin of the Mixteca terrane and/or from exhumed HP rocks. During the Permian, arc-related intrusions in both the Mixteca and Oaxaquia terranes were accompanied by dextral transtensional deformation and deposition of clastic rocks containing Permian detrital zircons and carbonates in periarc, pull-apart basins. Empirical relationships between the dip of the Benioff zone and the widths of arc and forearc indicate that the Permian trench lay beneath the eastern edge of the Mesozoic Guerrero terrane.