DO THE BILLECOCHA NORMAL FAULTS (ECUADOR) REVEAL EXTENSION DUE TO LITHOSPHERIC BODY FORCES IN THE NORTHERN ANDES

Active compressional and transcurrent structures are widely reported f or the entire northern Andes, whereas there is no clear evidence of ex tensional tectonics in that region. In contrast, both active compressi onal and extensional structures occur extensively in the Central Andes . Extensional tectonism in the central Andes occurs mainly in high pla teaus of large wavelength (greater than or equal to 100 km), and is in terpreted to have resulted from the effect of compensated high topogra phy at the lithospheric scale. The observation of Holocene normal faul ts on the low wavelength (less than or equal to 15 km) Billecocha high plateau in the northern Ecuadorian Andes raises a problem. Could the lithospheric body forces be the cause of the normal faulting in a chai n with a mean altitude of less than or equal to 2800 m? Compared to th e central Andes, where normal faulting is widely distributed across th e chain, normal faulting on the Billecocha plateau is concentrated in a restricted 4-km-wide zone. In addition, normal faulting does not ext end further than a few kilometres eastward, where compressive structur es have been observed. Considering the local character of the extensio n, body forces at the lithospheric scale cannot be responsible for thi s normal faulting, and thus we can preclude a significant lowering of the magnitude of sigma H-max in the northern Andes with respect to the central Andes. Analysis of these normal faults shows that normal faul ting occurred between 10,000 yr and 6000 yr B.P. In addition, the rela tionship between the geometrical parameters (vertical throw and ruptur e length) of the major normal fault differs significantly from that gi ven empirically by methods based on scale laws. in such a case, the Bi llecocha normal faults are unlikely to have a tectonic origin. Various local processes (e.g., gravity gliding, ''sacking'') that may be poss ible are contemplated for triggering the normal faulting. In addition, a theoretical simple 2-D model is proposed in where local conditions peculiar to the plateau (structure, glaciation) and the surrounding ar eas are taken into account. In this model, normal faulting occurs by s hearing along the planes of weakness (bedding planes) in response to t he elastic rebound of the fluid-saturated plateau, which prevailed aft er the last glaciation.