STRUCTURAL CHARACTERISTICS OF PINAL SCHIST IN THE EASTERN PART OF SIERRA DE LOS AJOS, NORTHEASTERN SONORA, MÉXICO

The Sierra de Los Ajos, in northeast Sonora, México, is composed by several rock sequences that includes the crystalline basement of proterozoic rocks assigned to Pinal Schist (1715 Ma) and Mesteñas Granite (1400 Ma); the paleozoic sequences as the Bolsa Quarztite (Cambrian) at the base, and the Escabrosa Limestone (Mississipian) in the upper part of the Paleozoic rocks; and the Mesozoic rocks composed by a Jurassic granite, by The Tuli Formation and by the Cabullona Group of Late Cretaceous . The stratigraphic relations between these rocks of this area are the following: The basement is constituted by the Pinal Schist and Mesteñas Granite where the last intrudes the metamorphic rocks; the paleozoic sequences cover by an angular unconformity to the proterozoic rocks; the Jurassic granite is intruding both proterozoic and paleozoic sequences; the sedimentary and vulcanosedimentary sequences of the Late Cretaceous, and the The Tertiary rocks cover unconformably to all the package of rocks described. Structurally, the sequences exposed in the Sierra de Los Ajos seem to form part of what it has been called the Cananea High and the fault -Los Ajos Fault- that puts in contact rocks of the El Tuli Formation with the proterozoic and paleozoic rocks is a major structure that can be the Cananea High northeastern structural limit. The eastern portion of the area studied (Sierra Las Mesteñas) represents a block trending NNW. This block corresponds litologically with the oriental portion of the Sierra de Los Ajos and can be part of other blocks that have been described some tens of kilometers to the north and southeastern of the area of study. The presence of faults is minor, being observed some normal faults affecting the sequence of the Late Cretaceous and the basement rocks. The protolith of the Pinal Schist was characterized like eugeosyncline rocks which were metamophosed around 1650 Ma. The foliation observed in the Pinal Schist for the area of study is predominantly tending NE and dipping to the N and NW. The Pinal Schist has several degrees of metamorphism and deformation for which in some outcrops can be observed possible relictic original bedding.