FRACTURE CHRONOLOGY AND NEOTECTONIC CONTROL OF WATER WELLS LOCATION IN CRYSTALLINE TERRANES: AN EXAMPLE FROM THE EQUADOR REGION, NORTHEASTERNMOST BRAZIL

Abstract

The brittle deformation in the Equador area, in the center-south domain of the Rio Grande do Norte State, NE Brazil, was analyzed to establish the chronological sequence of the different tectonic episodes and their stress fields, emphasizing the neotectonic activity by structural and morphotectonic criteria. The aim of this study was to test the relationship between local and/or regional neotectonic stress field and the yield of water wells drilled in the region. The local neotectonic structures and morphotectonic features agree with the regional stress field, defined by E-W compression (s 1 or S H ) and N-S extension (s 3 or S h ). The current kinematics of the fracture sets, with an “open” or “closed” behavior, seems to be related to the neotectonic stresses and to be highly influential upon the water-yielding potential of the fissural aquifer. Besides other controls, water wells located along roughly E-W trending fractures that act as extension joints have a higher hydrological potential than those along N-S fractures, which tend to be “closed”. NE-or NW-trending old fractures tend to reactivate by shearing. In this region, EW tension joints generated at end of the Brasiliano Cycle (filled with pegmatite or quartz veins), were reactivated under the N-S to NNW extensional deformation related to rifting and development of the Cretaceous basins. The same N-S extension direction is once again active as part of the Holocene stress field. In the Caiçara area, 2 km NW of Equador, the wells drilled in quartzite and metaconglomerate, located along E-W (± 20 o ) extension fractures and NE or NW shear fractures have been pumped in a full-time regime since the last 20 years, without major variations in their dynamic levels. On the other hand, wells located along longitudinal, N-S fractures or in poorly fractured sites frequently have very low yields.