Estilo estrutural e contexto tectonoestratigráfico do Grupo Ubajara no NW do Ceará

Abstract

The Ubajara Group is a neoproterozoic (of ediacaran age) low grade metassedimentary unit (ediacaran) that occurs adjacent to the TransbrasilianoKandi Lineament, in NW Ceará. This lineament corresponds to a NE-trending dextral strike-slip mega-shear zone related to the Brasiliano orogenic cycle. The Ubajara Group is part of the Middle Coreaú Domain, being completely surrounded by medium to low temperature shear zones that define its contact either with the Martinópole Group (also of neoproterozoic, tonian age, deformed at a higher strain and metamorphic grade) and gneissic-migmatitic terrains (paleoproterozoic to archaean). In its SE border, the Ubajara Group is in contact, through a NE-trending dextral transcurrent fault, with the ediacaran-cambrian Jaibaras Group, that fills the homonimous graben. The essentially tectonic contacts make it difficult to interpret the stratigraphic-structural context of the Ubajara Group and the metamorphic-structural contrast with the Martinópole Group and the gneissic-migmatitic terrains is evident. In addition, the absence of a nonconformity at the base of the Ubajara Group poses the hypothesis of expressive alloctony of this unit, preceeding the deposition of the Jaibaras Group. The Ubajara Group is involved in a complex deformation pattern with conytractional ans transcurrent structures related to the Brasiliano Cycle. The present work focuses on the structural style and tectonostratigraphic relations of the Ubajara Group with other adjacent units, through the interpretation of remote sensing imagery, field and microscale structural data. The second phase (D2) comprises ductile, transcurrent structures with dextral kinematics, including NEtrending steep-dipping shear zones, low-rake stretching lineations and open to tight folds with (S2) foliation trends varying from NS to NNE. The horn shape of the Mucambo Granite denotes the control of this phase during its emplacement. Finally, a late stage evolution to ductile-brittle or brittle structures also shares the same dextral transcurrent kinematics that characterizes the D2 phase (being thus referred as D2t). It is expressed by NE-trending normal dextral faults and E-W to ENE normal faults and extensional joints, being correlated to the transtensional tectonics that controls the Jaibaras pull-apart graben. In some mylonitic bands it is observed the raising of metamorphic grade. The main belt produced mylonitization in originally E-W trending subvolcanic granite dikes that filled extensional joints related to a transtensional termination site close to the Meruoca Pluton. The outstanding metamorphic-structural contrast of the Ubajara Group, as regards to the older units, as well as the lack of a non conformity in its base, point to an expressive alloctony, combining (D1) tangential transport and (D2) dextral transcurrent displacements, the latter associated with LTB. Finally, the final site of occurrence of the Ubajara Group was shaped by (D2t) transtensional structures correlated to those that shaped the jaibaras Graben.