LOW ANGLE SUBDUCTION IN NORTHEASTERN MÉXICO DURING EOCENE TIME

The end of the Laramide orogeny and the change from subduction-related to intraplate type magmatism in central Coahuila, México, have been documented and analyzed. The results of a structural, geochemical and geochronological investigation in central Coahuila, suggest that the Laramie deformation occurred in two stages. The first pulse occurred prior to 46 My b.p and triggered thin-skinned deformation controlled by detachments along evaporitic horizons in the Upper Mesozoic sequence. The second pulse reactivated basement major faults (i.e. San Marcos fault) and subsidiary structures. The age of deformation was dated using plutonic emplacement-deformation relationships between the Candela-Monclova Intrusive Belt (CMIB: 45 – 35 My b.p.) and laramidic structures. Shallow magma emplacement was controlled by reactivation of basement structures during the late stage of the laramidic orogeny. As a consequence, some intrusives display syn-tectonic features and others are post-tectonic. The isotopic ages, cross-cutting relations, and internal fabrics in the intrusive bodies indicate that the Laramide orogeny ended 41 My b.p. in central Coahuila. The CMIB shows two subduction-related magmatic series, one calc-alkaline with medium- to high-K content, and other alkaline. The isotopic age in the CMIB ranges from 45 to 35 Ma (Ar-Ar) and is older than the alkalic intraplate-type rocks found at Sierra de Picachos and La Cueva Intrusive Complex. The Sr, Nd and Pb systematics indicate that: 1) the magmas show low cortical contamination, 2) the subduction-related calc-alkaline and alkaline rocks were formed in different reservoirs in the mantle and, 3) the isotopic characteristics of the CMIB magmas result from mixing of three sources: MORB, upper-mantle and subducted sediments. The source change, from subduction-related to intraplate-type magmas, occurred in the lapse between 32.5 and 30 My b.p. The general conclusion indicate that the arc magmatism and the style of deformation related to reactivation of basement faults in the southern Coahuila Folded Belt, located ~700 km from the trench in the Eocene time, should had been controlled by low angle subduction of the oceanic Farallon under North American plate.