Development of a self-similar strike-slip duplex system in the Atacama Fault system, Chile

Abstract

Fault development models are crucial to predict geometry and distribution of fractures at all scales. We present here structures related to the development of the Bolfín Fault in the Atacama Fault System (AFS), covering a range of scales of 7 orders of magnitude. The AFS is a 1000km-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 >45km. It cuts mainly meta-diorites and exhibits a 100–200m thick core of subvertical bands of altered fractured host rock and of foliated cataclasites. This foliation is made up of several trend-parallel cm-thick shear bands, composed of plagioclase fragments (>0.1mm) surrounded by epidote.In the compressive quadrant around the tip point of Bolfín Fault, the lower strain faults exhibit an unusual internal structure consisting of fractures arranged in a multi-duplex pattern. This pattern can be observed from meters to millimeters scale. The fractures in the strike-slip duplex pattern can be separated into two types. Main Faults: trend-parallel, longer and with larger offsets; and Secondary Fractures: sigmoid-shape fractures distributed in the regions between Main Faults, all oriented between 15° and 75° with respect to the Main Faults, measured counterclockwise (i.e. in P-diedra).On the basis of the distribution of the two types of recognized fractures, the relative sequence of propagation can be inferred. Main Faults, the more widely distributed, propagated earlier. The Secondary Fractures, in turn, distributed in thinner areas between the larger Main Faults, were propagated later as linking fractures.The duplex pattern is self-similar: Multiple-Core Faults with internal structure of multiple-duplex are itself in turn secondary faults within a larger km-scale duplex (Caleta Coloso Duplex).The duplex width (W) and the length (L) of the Main Faults forming the duplex show an almost linear relationship, for duplexes observed from micro- to regional scale. For a scale range of seven orders of magnitude, the length of the master strike-slip faults is about ten times the width of the overlapping duplex area. This relationship evidence the self-similar nature of the structural system under study and suggests that the same fundamental mechanical processes that build up the small strike-slip duplexes operate also at the regional scale.