RECENT CRUSTAL DEFORMATION IN THE ANTOFAGASTA REGION (NORTHERN CHILE)AND THE SUBDUCTION PROCESS

New neotectonic observations, along with a detailed aerial photograph analysis, allow a new Interpretation of the recent tectonic behaviour of the outer forearc in northern Chile between 22.5 degrees S and 24.5 degrees S (Antofagasta region). Both the Coastal Cordillera and the M ejillones Peninsula are under E-W extension. Normal faults dipping: ea st with an almost N-S orientation are predominant, Large-scale Neogene to Recent deformation is characterized by vertical uplift and subside nce related to normal faulting. Recent kinematics along the Atacama Fa ult System, including the Atacama Fault itself, are controlled by the regional extensional stress regime, and the predominant component or r ecent displacement along the Atacama Fault is vertical (normal). Howev er, strike-slip components are also observed, but with moderate offset s, no larger than one to two tens of metres. The sense of shear, and t he ratio between vertical and horizontal offsets, vary coherently with the fault azimuth, compatible with E-W extension. Oblique-slip faulti ng, with normal and left-lateral offsets of similar amplitude, is obse rved on the most northeasterly oriented segment of the Atacama Fault. This small left-lateral component of displacement and the strike-slip motion predicted by models of strain partitioning of the convergence a long the Atacama Fault are in opposite senses, This occurs because the stress regime in the coastal region is not compressional but extensio nal. Very fresh recent ruptures are numerous, especially between 23 de grees S and 24 degrees S, and provide clear indications oi the occurre nce Of moderate to large continental earthquakes during the Late Quate rnary, As pointed out by several authors, indirect lines of evidence s uggest active subduction erosion and underplating along the margin of northern Chile. We propose that part of the long-term extension is due to the broad-scale flexure of the outer forearc. This flexure may be controlled by offshore subsidence caused by subduction erosion near th e trench, and by onshore uplift related to the underplating of eroded low-density material beneath the Coastal Cordillera, Neotectonic obser vations show that the state of stress in the outer forearc remains ext ensional (in a deviatoric sense) in the long term. However, it is prop osed that the state of stress has a strong time-dependent component, w hich is intimately linked to the subduction seismic cycle. The M-w = 8 .0 Antofagasta earthquake of 1995 July 30 showed that large subduction earthquakes produce E-W extension in the coastal region, The overall E-W deviatoric er;tension in the outer forearc should be reduced by in terseismic contraction in the period separating two subduction earthqu akes, This reduction means that continental faults will remain locked and aseismic in the interseismic period of the subduction cycle, in ag reement with microseismic observations, which indicate an absence of s hallow crustal seismicity, We assume that the amount of extension prod uced by subduction earthquakes in the coastal area is larger than the cumulative interseismic contraction. This occurs because the displacem ent field generated by subduction earthquakes is concentrated in the o uter forearc. The E-W extension should grow over repeated subduction s eismic cycles and lead to large continental earthquakes occurring with in the overriding block, simultaneous with large subduction events. We show new surface ruptures associated with normal faulting along the A tacama Fault, which were most probably triggered by the 1995 Antofagas ta subduction earthquake.