Structural analysis of two juxtaposed jurassic lithostratigraphic assemblages in the Sierra Madre Oriental fold and thrust belt of Central México

Abstract

A general comparison of three neighboring areas in the Sierra Madre Oriental of central Mexico, in the east of the Queretaro State, suggests the existence of a Mesozoic continental crust bordering a subduction zone. East of the Higuerillas Thrust there is a 2400 - 3 500 m thick outcrop of marine deposits composed of (I) Upper Jurassic - N eocomian shaly limestone and shale; (2) Middle Cretaceous limestone, and (3) Upper Cretaceous pelitic rocks. West of the Higuerillas Thrust, two lithotectonic assemblages are juxtaposed: the westerly one con.tains in its lower part intercalations of fme grained micaceous greywacke within phyllitic shale, volcano-sedimentary rocks, and radiolarian-ribbon chert, related to a magmatic arc. The age of these lithologies could be Middle to Late Jurassic. Following up-section and separated by an angular unconformity, there are shallow marine limestone, conglomerate, sandstone, • shale and red beds ofNeocomian age. The eastern assemblage consists of coarser siliciclastic and alkaline volcanoc1astic rocks of late Kimmeridgian to Tithonian age. The sequence east of the Higuerillas Thrust extends over a larger area and is more representative of the lithology of the Sierra Madre Oriental fold-and-thrust belt. The siliciclastic rocks were probably deposited when the former North American shelf was being cut by high-angle normal faults, which may still have been active from the Neocomian to the Barremian time. The Jurassic assemblage outcropping more westerly of the Higueri1las thrust include in situ disrupted and fragmented greywacke, tuff and radiolarian ribbon chert in a muddy matrix and display frequently penetrative cleavage dipping 0 - 400 to the southwest. This deformation style is nearly absent in the coeval coarser rocks placed to the east. The limit between the two assemblages is defined by the El Front6n Fault Zone, which consists of a series of imbricate thrust slices.