State of In Situ Stress in Northern Chile and in Northwestern Argentina

Abstract

In situ stresses have been determined at rock surfaces by the overcoring method at 45 locations in northern Chile and in northwestern Argentina between 22° and 26° S. From the Pacific coast to the Subandean Ranges we recorded the actual state of stress of all prominent morphotectonic units. There are four distinct regional stress fields: (1) The west Chilean stress field comprises the complete Coastal Cordillera between the Pacific coast and the western margin of the Longitudinal Valley. While horizontal tensile stresses oriented E-W prevail on the Mejillones Peninsula, along the coast north of Antofagasta and in the area of Taltal, horizontal compressive stresses (σHmax = σ1) directed NE-SW predominate in the entire Coastal Cordillera. σHmax reveals mean values of 9 MP, a whereas σHmin (= σ2) oriented NW-SE yields — 1 MPa on average. In situ stresses of the west Chilean stress field signify an actual uplift of the Coastal Cordillera bordered on the west (Mejillones Peninsula) and on the east (Atacama Fault) by active normal faults striking N-S. (2) The Central Chilean stress field includes the Longitudinal Valley and the western part of the Chilean Precordillera. From west to east compressive aHmax directed NW-SE to N-S decreases from 40 to 7 MPa, whereas tensile σHmin oriented NE-SW to E-W increases from 0 to -7 MPa. Generally aHmax trending NW-SE corresponds to σ2 in the west and to al in the east of the Central Chilean stress field, indicating actual normal faulting in the west (Atacama Fault, Longitudinal Valley) and strike-slip or thrust faulting in the east (Calama). (3) The Central Andean stress field extends from the Preandean Cordillera Domeyko across the Preandean Depression, Western Cordillera, and Puna, and includes the western area of the Eastern Cordillera. Maximum horizontal in situ stresses are generally oriented E-W yielding average values of 14 MPa (N-S, σHmin = 0.7 MPa) in the western part of the Central Andean stress field (Cordillera Domeyko, Preandean Depression, Western Cordillera), whereas in the Puna lower E-W compressive stresses only occasionally exceed 10 MPa. Horizontal tensile stresses oriented NW-SE and of -5 to -15 MPa along the Olacapato — El Toro lineament indicate its probable recent strike-slip movement. (4) The east Andean stress field is defined by the eastern part of the Eastern Cordillera or may extend to the east into the Subandean Ranges. Maximum horizontal stresses are compressive in the NE-SW direction and vary between 2 and 17 MPa. In situ horizontal stresses across the uppermost crust of the Andes between 22° and 26° S are dependent on the thickness of the rigid Andean crust rather than on topographic elevation.