MAPPING AND KINEMATIC ANALYSIS OF THE SIERRA EL COBRE: A NEW LOCALITY OF METAMORPHIC CORE COMPLEX(?) IN NORTHWEST SONORA, MEXICO

New mapping in the Sierra El Cobre lies in northwest Sonora reveals a country rock framework of dominantly Mesozoic and Tertiary rocks. These rocks conform a high topographic relief that comprise an arch of distinctively deformed and metamorphosed igneous and sedimentary rocks.

The Mesozoic rocks include granodioritic plutons and unmetamorphosed clastic rocks as sandstone, siltstone, conglomerate, and andesitic to tuffaceous rocks that range from Middle Jurassic to Late Jurassic. Tertiary rocks, that include mylonitic tectonites formed at the expense of granitic and conglomerate protolith, are the main body of Sierra El Cobre.

The strongly deformed mylonitic tectonites composed of low to medium grade metasedimentary and metaintrusive rocks underlie major north-trending sierra. Doubly plunging foliation defines an antiform flexure, the El Cobre antiform, bounded to the east by the El Cobre fault, an inverse or a detachment system (?). The lower plate is strongly foliated, composed of metaconglomerates and schist; metaconglomerates records strong deformation suggestive of a planar fabric, flattening. Schistosity dips >40º and it is cut by sinistral kink folds and S shaped folds. El Cobre fault zone is marked by intense shearing where kinematic indicators suggest a transport to the west with steeply dips 40-70º.

Data from Sierra El Cobre suggests that it represents a tectonic corrugation associated to the evolution of the El Cobre fault. Controversy come up if this fault is initially a single normal fault or a reverse fault as part of fault system of the metamorphic core complexes development.