Arizpe sub-basin: A sedimentary and volcanic record of Basin and Range extension in north-central Sonora, Mexico
AbstractThe Arizpe sub-basin located in the northern part of the Río Sonora basin is a Basin and Range half-graben that initiated during Late Oligocene time in north-central Sonora. Its ~2.1 km-thick, east-dipping volcanic and sedimentary fill assigned to the Báucarit Formation is divided, from base upwards, into the following informal members. The La Cieneguita member composed of interbedded conglomerate, siltstone and gypsum beds which unconformably overlay older Cenozoic volcanic rocks; the El Toro Muerto basalt composed of basalt flows, basalt breccia and subordinate conglomerate beds; the Arzipe conglomerate composed of three fining-upwards conglomerate sequences that interdigitates with flows of the Tierras Prietas basalt in its lower part and the Agua Caliente basalt in its upper part; the Bamori member is a coarsening-upward succession of siltstone, sandstone and conglomerate that unconformably overlies the Arizpe conglomerate and it is unconformably overlain by the sedimentary El Catalán breccia. Basin accommodation started at ~25 Ma when deposition of the La Cieneguita member, followed by alkaline basaltic volcanism of the El Toro Muerto and contemporaneous rhyolitic volcanism, floored the area predating significant clastic deposition. The Agua Caliente basalt (~21 Ma ) in the upper part of the basin fill indicates the basin was rapidly subsiding. Multiple phases of normal faulting affected the Arizpe sub-basin. The main controlling structure may be the steep (80°), west-dipping, sub-parallel El Fuste and Granaditas normal faults that bound the Arizpe sub-basin at its present-day eastern margin, or there may be a fault or faults that were subsequently buried beneath younger basin fill near the eastern margin of the basin. The basin was disorganized by an even younger NW-SE phase of normal faulting represented by the southwest-dipping Crisanto and Tahuichopa faults. Growth strata within basin fill suggests that syntectonic deposition was active during all phases of normal faulting. However, punctuated tectonic activity on these faults may have controlled deposition of conglomerate sequences of the Arizpe conglomerate.
Geochemical data from the El Toro Muerto, the Tierras Prietas and the Agua Caliente basalt members indicate they are high-K, alkaline to subalkaline basaltic trachyandesites with light REE-enriched patterns, initial Sr ratios between 0.7069 and 0.7076, and εNd values between -3.76 and -4.88. Pb isotopic values from two samples of the El Toro Muerto basalt yielded very similar results, and along with the other geochemical data suggest an important participation of the continental lithosphere as magma source for this volcanism.
The data herein reported are supported by eight new geochronologic ages and they contribute to better document and constrain the timing of magmatism and extension in the Basin and Range tectonic province in Sonora.
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