Internal Structure and Microtextures of a Quaternary Upper-plate Fault Zone: A Case Study from the Atacama Fault System, Northern Chile.

Quaternary deformation in the northern Chile coastal forearc is mainly accommodated by ubiquitous upper-plate faults cataloged as weak fault zones, however, the deformation mechanisms and the internal structure of these reactivated faults remain poorly understood. To solve this problem, we selected seven study sites from reactivated upper-plate faults of the northern Chile forearc (23-25°S). These faults formed during the Early Cretaceous and reactivated during the Quaternary forming conspicuous fault-scarps. Here we present a new characterization of the internal structure at the outcrop and microscopic scale. Samples for thin-sections and XRD were collected in several cross-sections across faults. We define 4 units conforming the internal structure: (1) A decimetric well-defined principal slip zone, referred here as active fault core (AFC), consisting of a gouge layer subunit bounded by a fault breccia subunit, (2) a metric inactive fault core (IFC), surrounding the AFC, composed mostly of cataclasites and in some cases, mylonites, (3) a host-rock unit corresponds mainly to Jurassic-Cretaceous dioritic-granitic intrusives and Jurassic andesites, and (4) a decametric damage zone affecting both the IFC and the host rock. Near the topographic surface, the gouge layer subunit consists of a grey/green ultrafine matrix (40-80%) partially to completely replaced by massive iron oxides. In some sites, the gouge layers are partially foliated or/and exhibit millimetric bands of chaotic microbreccia. Porphyroclasts correspond mainly to (1) highly quartz and plagioclase intracracked individual crystals (<0.4mm), (2) larger fragments (<1mm) generally sigmoidal-like of the IFC (cataclasites) indicating different degrees of cataclastic-flow. Transgranular microfractures are densely propagated through the boundaries of larger porphyroclasts, breaking grains into ever-finer fragments (constrained communition) and generating chaotic microbreccia halos in the boundaries that grade into an ultrafine gouge matrix. (3) Another portion of large porphyroclasts (>1mm) grade from S-C cataclasite at its cores to S-C ultra-cataclasites at its boundaries. Frictional sliding is propagated through this S-C fabric formed by the ultracataclasite boundaries, generating well-defined and smoothened surfaces between large porphyroclasts and gouge layers. Microfractures -commonly filled with quartz>calcite>albite>chlorite-epidote veins- propagate mostly through the gouge layers, which are in turn displaced by microfaults affecting the entire subunit. The IFC composition changes markedly along-strike but multiple-fault cores are ubiquitous. In Jurassic andesites, the IFC is defined by protocataclasites with layers of red gouge, In Jurassic to Cretaceous diorite-metadiorite protoliths the IFC is defined by S-C cataclasites with microstructures showing undulating extinction, subgrains, and bulging recrystallization of quartz, and ultracataclasite bands and green gouge layers developed under low greenschist facies conditions. The IFC formed in mylonitic rocks derived from Jurassic to Cretaceous granitoid includes bands of hydrothermally-altered green and red mylonites. The complex overprinted microtextures indicate a progressive exhumation and shearing of the IFC. The microtexture analysis reflects the evolution of this unit from high temperature-low stain rates formed at deep structural levels to low temperature-high strain rates near-surface. We interpret the highly accumulated strain in S-C ultracataclastic bands and S-C gouge layers of the IFC (constrained communition) reduces the fault frictional strength and promote the frictional slip of the quaternary reactivations of the AFC.