ALBERT

Description: Late Miocene plutons in coastal Chiapas, Mexico, represent the roots of an extinct magmatic arc. Miocene granitoids of calc-alkaline composition and arc chemistry intruded into and were deformed within the Tonalá mylonite belt in the middle to upper crust. The mylonite belt is a crustal-scale shear zone extending along the western margin of the Chiapas Massif for ~150 km. Deformation is characterized by a dominantly subhorizontal lineation and subvertical foliation along a strikingly linear zone that trends ~310°. Mylonitic fabrics contain ambiguous but dominantly sinistral shear indicators. Intrusions are interpreted as syntectonic on the basis of similar U-Pb zircon crystallization age estimates (ca. 10 Ma) and the cooling age estimates obtained on neoformed micas in the mylonite. The plutons are elongated, their long axis is parallel to shear zone, and some plutons show markedly asymmetric outcrop patterns, with sheared tails that trail behind the intrusions and that are consistent with sinistral displacement. Parts of plutons were mylonitized by continuous deformation in the Tonalá shear zone, locally developing intricate pseudotachylyte and cataclasite veins slightly oblique to the mylonite foliation. Outside of the shear zone, plutons preserve magmatic fabrics. These observations are consistent with features common to syntectonic granites interpreted to have been emplaced along strike-slip shear zones in a transpressional setting. We interpret the Tonalá mylonites as representing a relict transform boundary that was slightly oblique to the Polochic-Motagua fault system, which accommodated over 100 km of sinistral displacement between the Chortis block (on the Caribbean plate) and Chiapas (on the North America plate) in late Miocene time.

Description: Rift-related magmatism in the northernmost Gulf of California and the adjacent subaerial Salton Trough and Cerro Prieto basins comprises intermediate to rhyolitic surficial and buried lava flows and domes, including their xenolith cargo. In addition, geothermal drill wells frequently penetrate subsurface gabbroic to granitic sills and dikes, which intruded into Colorado River delta fluviatile and lacustrine sediments. Combined single-crystal U-Th-Pb and (U-Th)/He zircon ages reveal late Pleistocene to Holocene eruption ages for three volcanic centers in adjacent rift basins (from N to S): Salton Buttes (eruption age: 2.48 ± 0.47 ka; 95% confidence), Cerro Prieto (maximum eruption age: 73 ± 7 ka), and Roca Consag (eruption age: 43 ± 6 ka). U-Th zircon and allanite crystallization ages are close to the eruption ages, with the exception of Roca Consag lava, the zircon population of which is dominated by zircon with ca. 1 Ma crystallization ages, a population interpreted to be recycled from an unknown crustal source underlying the Wagner basin. Nd isotopic ratios for subsurface microgabbros from Cerro Prieto ( Nd = +8.9) overlap with values for mid-oceanic-ridge basalts (MORB) from the East Pacific Rise, adjacent to the southern Gulf of California. Cerro Prieto microgranites and Salton Sea basaltic xenoliths have similarly elevated Nd values. The lowest Nd value for late Pleistocene–Holocene igneous rocks from the northern Gulf of California is for Cerro Prieto dacitic lava ( Nd = +0.6). This value implies minor (〈20%) assimilation of continental crustal rocks, which, however, is an upper limit because of crystal-scale evidence for magma contamination by unconsolidated sediment at the time of eruption. Zircon crystals in felsic rocks (rhyolite lavas, intrusive microgranites, and granophyre xenoliths) have trace-element and submantle 18 O compositions that are robust indicators for a mafic source that has exchanged oxygen by interacting with meteoric hydrothermal fluids. Collectively, these data imply that oceanic rifting has initiated in the Salton Trough and Cerro Prieto basins. There, MORB-type magmas formed mafic intrusions within thick sedimentary basin fill, where they became exposed to deep-reaching hydrothermal fluids. Diverse intermediate- to high-silica rhyolitic magmas that are prevalent at the surface are produced by fractional crystallization of mafic parental magmas with minor assimilation of sediments or pre-rift basement rocks, and by partial melting of hydrothermally altered mafic intrusions.