Uplift and syn-orogenic magmatism in the Concepción del Oro Block: A thick-skinned (Laramide style?) contractional structure in the Mexican Fold-Thrust Belt

Abstract

In this paper we report UPb zircon ages from a syn-tectonic magmatism related to a contractional thick-skinned deformation pulse in the Mexican Fold and Thrust Belt. The study area corresponds to the Concepción del Oro Block (COB), which is located in northern Zacatecas, Mexico. The COB is a NW-SE trending elongated tectonic structure located south of the Parras Transversal Sector in the Mexican Fold Thrust Belt (MFTB) in north-central Mexico. In this area, regional thin-skinned folds were formed between the late Turonian and Campanian and they define the core of a major northeastward-convex orocline around Concepción del Oro mining district, Zacatecas, Mexico. The regional folds were cut by four Eocene-early Oligocene plutons (43-32 Ma; UPb zircon), which in places modified the older thin-skinned structures. Magma was emplaced along thrust faults and modified the attitude of inverted limbs of overturned folds, and the orientation or position of the fold axis at the core of anticlines, causing the formation of complex emplacement-related deformation. Two NW-SE trending lineaments bound the COB, and both cut the periclinal ends of the thin-skinned older folds, at different time. The eastern lineament is here interpreted as a high angle reverse fault, which tilted polymictic conglomeratic continental succession that includes cobbles derived from one of the intrusive bodies and an andesitic syn-tectonic lava flow (40.7 ± 0.6 Ma; UPb zircon). The western lineament is a normal fault generated by later regional tectonic extension related to the Basin and Range province. Altogether suggests that a late Eocene-early Oligocene contractional thick-skinned deformation pulse affected the COB. This pulse was contemporaneous with the Eocene-Oligocene magmatism in the Concepción del Oro, Zacatecas region, and is similar in style and age to some of the structures produced by the Laramide orogeny in the southern Rocky Mountains and in some isolated areas in northern Mexico.