Abstract

Fault development models are very important to predict geometry and distribution of fractures at all scales, however models based on structures from microns to km are relatively scarce, due to the lack of well-exposed structures. We present structures related to the development of the Bolfín fault in the Atacama Fault System (AFS), covering a range of scales of 9 orders of magnitude. The AFS is a 1000 km-long trench-parallel fault system located in the Andean Forearc. The Bolfín fault is a first-order fault of the Caleta Coloso Duplex, has a trend ~170° and length >45 km (Fig A). It cuts mainly meta-diorites and exhibits a 100-200m wide core of subvertical bands of altered fractured host rock and of foliated cataclasites. This foliation is made of trend-parallel cm-wide shear bands, composed of plagioclase fragments (>0,1mm) surrounded by epidote. Around the bands there are many micro fractures oriented within the P-diedra. In the compressive quadrant around a tip point of Bolfín fault, the lower strain faults exhibit an unusual internal structure consisting fractures arranged in a multi-duplex pattern. This pattern can be seen from metric- (Parulo fault, fig C) to mmscale (Palmera fault fig B). The fractures in the pattern can be separated in 2 types: Main Faults: Trend-parallel, longer and with larger offsets. Secondary Fractures: sigmoid-shape fractures distributed in the regions between main faults, all oriented between 15° and 75° with respect to the main faults, meassured in the shearsense (i.e. in P–diedra). On the basis of the distribution of the 2 types of fractures, the generation sequence can be inferred. The main faults are more widely distributed, and were propagated earlier. The secondary fractures are distributed in thinner areas between the larger displacement main faults, and where propagated later as linking fractures. The duplex pattern is self-similar: Faults with internal structure of multiple-duplex are in turn secondary faults in a larger km-scale duplex (Caleta Coloso). This fact suggests that the fracturing oriented in the P-quadrant has controlled the fault propagation at all scales.