The relationships between volcanism and extension in the Mesa Central: the case of Pinos, Zacatecas, Mexico
Pinos volcanic complex is an uplifted area that exposes Mesozoic strata and mid-Tertiary volcanic and sedimentary rocks. Its stratigraphy, deformation style, and volcanism are characteristic of the Mesa Central region of central Mexico and the southeastern segment of the Sierra Madre Occidental (SMO) volcanic province. The oldest rocks in Pinos are marine carbonate sedimentary and siliciclastic rocks that underlie a red bed sequence (Pinos red beds) interlayered with felsic volcanic rocks, in turn partially covered by a voluminous lava dome complex. The Pinos red bed sequence is at least 900 m thick and it is formed by well-lithified conglomeratic sandstone and matrix-supported, generally fine-grained to medium-grained, polymictic conglomerate. Clasts in the Pinos red beds were derived from the Mesozoic basement, subaerial felsic volcanic rocks of unknown provenance, and tourmaline-bearing muscovite granite. Interlayered volcanic rocks include ash-fall tuffs, a densely welded ash-flow tuff, and water-laid or reworked pyroclastic material. The main components of the dome complex are a dark-red, porphyritic potassium-rich trachyte, and a buff-colored, porphyritic rhyolite, for which we report lava mingling (the first one in the SMO volcanic province). Field relations at the Pinos volcanic complex demonstrate a close temporal relationship between felsic volcanism and extension. Faulting in Pinos is complex as it includes arrays of Cenozoic normal faults with NS, NW, and NE trends, for which cross-cutting relations are ambiguous. A combination of mapping, K-Ar geochronology, petrographic work and interpretation of the magnetic polarity of the volcanic units allow us to establish that repeated pulses of synextensional volcanism occurred during the period between ~32 and 27 Ma. These data demonstrate that extension in the Mesa Central is older than 29–27 Ma, the oldest previously recognized episode of extension. The earliest (≥ 32 Ma) pulse of extension may be related to a regional (~250 km long) NW-trending fault system that divides the Mesa Central into two domains with contrasting stratigraphy and different geomorphic aspect. The Pinos red bed sequence includes clasts of the Peñón Blanco granite (40Ar/39Ar = 50.94 ± 0.47 Ma), that in addition to providing a minimum age for the end of the Laramide orogeny in the Mesa Central, offers evidence of uplift and denudation of the granite before or synchronous with red bed deposition.
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