Prebatholithic metasedimentary rocks east of El Marmol on the 30th parallel of Baja California include tightly folded Permian and Lower Triassic rocks metamorphosed to upper greenschist/amphibolite facies. These rocks are divided into one informal and three formal formations. The rocks of El Marmol (informal formation) have a minimum thickness of 2,000 m and consists of thin-bedded argillite, sandstone, and chert with sparsely fossiliferous carbonate rock interstratified with more thickly bedded lenses of calcareous quartzarenite, impure carbonate rock, and chert-quartzite-carbonate clast conglomerate. The rocks appear to have been variously deposited by sediment gravity flows and intervening quiet-water deposition in a hemipelagic setting. The rocks of El Marmol contains no datable fossil material but are Lower Permian or older. The Arroyo Zamora Formation has an estimated thickness of 500 m, conformably overlies the rocks of El Marmol, and is composed primarily of massive or crudely laminated silty metaargillite that locally exhibits fine-grained turbidite successions and pebbly mudstone. Slumps, boudined sheets, and channelized sediment, gravity-flow deposits composed of coarser carbonate and quartzose clasts, including carbonate blocks up to several meters, are scattered throughout. Fossils in both the matrix and clasts of these coarse-grained deposits include Lower Permian (Leonardian) fusulinids, bryozoa, brachiopods, and crinoid columnals. The Cerro El Volcan Formation has an estimated thickness of 1,400 m and is divided into three members. The lowermost member (A) consists of 1,000 m of rhythmically bedded, dark siliceous argillite that contains intervals of metamorphosed massive calcareous sandstone, sandy limestone, and thinly bedded carbonate rock with siliceous partings. The overlying member (B) consists of 50 m of metamorphosed conglomerate, pebbly quartzite, and calcareous sandstone. Angular blocks throughout this member commonly contain bioclastic debris. The uppermost member (C) consists of 350 m of laminated argillite containing widely spaced interbeds of metamorphosed argillaceous carbonate rock. It is considered Late Permian in age because it lies conformably on rocks of Early Permian age, and is overlain with possible disconformity by the Lower Triassic De Indio Formation. The De Indio Formation has an estimated thickness of 300 m and is divided into two members. The lower member (A) contains up to 50 m of metamorphosed chert-pebble conglomerate, quartzite, cross-bedded calcareous sandstone, and limestone. The upper member (B) consists of 250 m of carbonaceous, staurolite-bearing argillite. Ammonoids and conodonts from member A are of Early Triassic (Smithian) age. These four formations have lithologies and faunal assemblages similar to coeval rocks deposited along the outer edge of the Cordilleran miogeocline of western North America, and were probably deposited in a southern extension of this miogeocline.

ABSTRACT The Arteaga-Tumbiscatío region of the Zihuatanejo terrane forms the westernmost part of the Guerrero composite terrane. This region contains one of the most complete stratigraphic columns of the terrane. Its evolution is divided into nine main tectonic stages characterized by alternating deformational and volcanic-arc events. The oldest rocks in the region belong to the Arteaga Complex, which is made up of highly sheared and locally metamorphosed turbidites (shale, quartzose sandstone, and black chert) and volcanic siltstone that forms a sedimentary matrix with blocks of mid-ocean-ridge basaltic lavas, banded gabbro, green chert, and limestone. The sedimentary matrix is mostly made up of continent-derived quartzose sandstone that yields 1.2 Ga Nd model ages. The age of deposition of the Arteaga Complex is constrained by Triassic (Norian) radiolarians from chert, Permian–Triassic maximum detrital zircon depositional ages, and a U/Pb zircon age of 180 Ma from a block of gabbro. Lithologic associations, geochemical affinities, and contact relations suggest that rocks of the Arteaga Complex were originally deposited in an oceanic basin that evolved near the paleocontinental margin of Mexico-Gondwana (stage 1). The Arteaga Complex was deformed as a subduction-related accretionary prism sometime between the Late Triassic and Early Jurassic (stage 2, D1 and D2) and was intruded by Upper Jurassic granodiorite (stage 3, VA-I) that shows more evolved geochemical and isotopic signatures (peraluminous) than the subsequent magmatic events. These granitoids yielded a 40Ar/39Ar age of 152 ± 0.07 Ma and Nd model ages of 1.2 Ga and are interpreted as related to subduction initiation. Rocks of Lower Cretaceous mafic volcanic-arc affinities (stage 4, VA-II) are not exposed in the study area. Jurassic granodiorite intrusive units were deformed, mylonitized, and exhumed (stage 5, D3) previous to the development of a Cretaceous (Aptian–Albian) volcanic arc (stage 6, VA-III). Four Cretaceous lithostratigraphic units (stage 6, VA-III), the Agua de Los Indios, Pinzán, Resumidero, and Playitas Formations, unconformably overlie the Arteaga Complex and deformed Jurassic intrusive units. All were deposited in an intra-arc basin and record two alternating sea-level transgression/regression cycles. The Cretaceous succession was deformed, causing wide regional folds (stage 7, D4) that are interpreted as initial pulses of the contractional event that originated the Mexican orogen of the Sierra Madre Oriental fold-and-thrust belt of eastern Mexico. Other areas of the Zihuatanejo terrane recorded Santonian–Maastrichtian arc volcanism (stage 8, VA-IV) that was coeval to late pulses of the Late Cretaceous contractional event that formed the Mexican orogen. This was followed by Eocene magmatism and strike-slip deformation (stage 9, VA-V).