Paleomagnetism, magnetostratigraphy, provenance, and tectonic setting of the Lower Cretaceous of nuclear southern Mexico

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We studied the sedimentology, provenance, and paleomagnetism of Lower Cretaceous strata in nuclear southern Mexico deposited in continental, marginal and marine settings in order to better understand their tectonic setting and paleographic correlations. The studied units have been assigned to the Zicapa, Atzompa, Caltepec, Xonamanca, and Jaltepetongo formations, which are interpreted as an eastward-deepening paleogeography (in present day coordinates) deposited between Valanginian and Aptian time in a system of extensional basins contemporaneous with arc magmatism. Provenance interpreted from sandstone petrology, conglomerate clast counts, and detrital zircon U–Pb geochronology indicates that these successions consist of polymictic conglomerate, quartz-lithic metamorphoclastic and feldspathic-quartz lithic volcaniclastic sandstone, and mudstone. These were sourced from the underlying Precambrian and Paleozoic basement rocks, as well as from proximal suites of mafic to acid Jurassic (∼182-145 Ma), and Cretaceous volcanic rocks (∼140-123 Ma). The remanent magnetization in the Atzompa and Caltepec formations is multivectorial, and the characteristic remanent magnetization resides in hematite. For the Atzompa the grand mean is Dec = 335.3° and Inc = 25.4° (k = 15.4, α95 = 12.7°; N = 10 accepted sites). For the Caltepec the grand mean is Dec = 354.3°, Inc = 27.9° (k = 29.15, α95 = 6.1°, N = 20 accepted sites). Magnetizations in both units predate folding and pass reversal tests. For the uppermost Zicapa the magnetization is nearly univectorial and resides in a soft magnetic phase; the grand mean is D = 275.2°, I = 36.0° (k = 34.4, α95 = 10.1°, N = 7 accepted sites). For the Caltepec and Atzompa formations, I/E corrected directions indicate a paleolatitude of ∼20° to 23°N which is indistinguishable from that expected assuming stability with respect to the North America craton (∼24°), implying only minor tectonic motions since Early Cretaceous deposition, and demonstrating that the present distribution of units studied resembles their paleographic relations. The magnetostratigraphy of these units can be correlated with some confidence to the M10-M0r sequence of the geomagnetic polarity time scale, suggesting that the formations record late Valanginian and early Aptian marine transgressions that might correlate with global eustatic curves. The temporal and paleogeographic distribution of magmatism in southern Mexico is consistent with the presence of a continental arc formed by eastward subduction under the North American plate continuously from Early Jurassic to Aptian time, and extension in a back-arc setting during the Hauterivian-Barremian driven by slab rollback. We assign the system of extensional basins to the “southlands rift”, a short-lived event that predates amalgamation of oceanic terranes in the west and Albian carbonate platform development.