Do the Billecocha normal faults (Ecuador) reveal extension due to lithospheric body forces in the northern Andes?

Abstract

Active compressional and transcurrent structures are widely reported for the entire northern Andes, whereas there is no clear evidence of extensional tectonics in that region. In contrast, both active compressional and extensional structures occur extensively in the Central Andes. Extensional tectonism in the central Andes occurs mainly in high plateaus of large wavelength (≥ 100 km), and is interpreted to have resulted from the effect of compensated high topography at the lithospheric scale. The observation of Holocene normal faults on the low wavelength (≤ 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 chain with a mean altitude of ≤ 2800 m? Compared to the central Andes, where normal faulting is widely distributed across the chain, normal faulting on the Billecocha plateau is concentrated in a restricted 4-km-wide zone. In addition, normal faulting does not extend further than a few kilometres eastward, where compressive structures have been observed. Considering the local character of the extension, body forces at the lithospheric scale cannot be responsible for this normal faulting, and thus we can preclude a significant lowering of the magnitude of σH max in the northern Andes with respect to the central Andes. Analysis of these normal faults shows that normal faulting occurred between 10,000 yr and 6000 yr B.P. In addition, the relationship between the geometrical parameters (vertical throw and rupture length) of the major normal fault differs significantly from that given empirically by methods based on scale laws. In such a case, the Billecocha normal faults are unlikely to have a tectonic origin. Various local processes (e.g., gravity gliding, “sackung”) that may be possible 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 areas are taken into account. In this model, normal faulting occurs by shearing along the planes of weakness (bedding planes) in response to the elastic rebound of the fluid-saturated plateau, which prevailed after the last glaciation.