Paleomagnetism, structure and 40Ar/39Ar geochronology of the Cerro Mercado pluton, Coahuila: Implications for the timing of the Laramide orogeny in northern Mexico
The Cerro Mercado pluton, emplaced in the Coahuila fold belt, contains magmatic, ductile, and brittle fabrics that suggest local and regional deformation during igneous emplacement, later affected by regional deformation in the area. The Cerro Mercado pluton intruded, deformed and uplifted Upper Cretaceous strata in the southern edge of the central depression of the Sabinas basin. A hornblende mineral separate from the pluton yielded a 40Ar/39Ar plateau age of 44.29 ± 0.19 Ma (1σ), interpreted as an approximation to the crystallization age of the monzonite. A younger biotite total fusion age of 41.23
± 0.21 Ma (1σ) from a different rock sample is interpreted to represent slow cooling (~80°C/Ma) of the Cerro Mercado pluton. Paleomagnetic analyses of the pluton yield well defined remanent magnetizations of nearly uniform reverse polarity, with a grand mean of D=178.2° and I=-61.7° (N=9 acceptable sites; k=57.5, α95 = 6.8°). These data are discordant with respect to the Eocene reference direction, indicating apparent clockwise rotation (~8°) and inclination steepening (~21°). The simplest interpretation of the discordance is that northwestward tilting of the pluton occurred during basin inversion within a weakly right lateral transpressive regime with NNW directed contraction. The strain field is consistent with contraction during the Laramide orogeny. Also, remanence acquisition is contemporaneous with both tectonic and magmatic fabrics in the pluton indicating that it is syntectonic. This suggests – in agreement with stratigraphic data – that the Laramide orogeny in central Coahuila ended some time after about 44 Ma. We propose that reactivation of the San Marcos fault and other basement faults in the Sabinas basin represents the last manifestation of shortening produced by the Laramide orogeny. This was a short episode of deformation that ended by the time of emplacement of younger plutons of the Candela- Monclova magmatic belt ca. 41 Ma. This indicates that culmination of the Laramide orogeny in central Coahuila is only slightly younger than in central Chihuahua.
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