Geometry and development of a hybrid thrust belt in an inner forearc setting: Insights from the Potrerillos Belt in the Central Andes, northern Chile

摘要

Thick- and thin-skinned hybrid thrust belts are typically developed in areas where crustal shortening was preceded by extension and normal faulting. They have been recognized in many places within the Central Andean backarc regions of Peru, Argentina, and Bolivia. In contrast, these structures have not been broadly studied in forearc regions. To understand the geometry and kinematics of this style of tectonism in a forearc setting related to Andean-type subduction zones, we used field and industrial 2D seismic data to study the Potrerillos thrust and fold belt and the Salar de Pedernales Basin in northern Chile (26 degrees S). Our results indicate that the structure in this region consists of a dominantly east-verging contracting system composed of large basement thrusts, Mesozoic inverted normal faults, and shallow thrust-related folds. Large basement thrust ramps and imbricated wedges represent the most prominent basement structures, which accommodated major crustal shortening. Reactivated and tectonically inverted Mesozoic extensional structures also constitute an important structural component, especially beneath the Salar de Pedernales Basin. These exhibit asymmetrical inverted anticlines with arrowhead shapes on the hanging walls of the inverted normal faults. The subsidiary shallow thrust-related folds commonly display fault bend and propagation folds that primarily accommodated the shortening of the Mesozoic and Cenozoic stratified deposits overlying the basement thrusts and cored anticlines. The age of the contraction in this region is poorly constrained; however, many of the oldest sequences accumulated in the top of the syn-rift sequences and related to synorogenic deposits, can be correlated with those identified in neighboring regions (e.g., Salar de Atacama, Salar de Punta Negra basins and Frontal Cordillera, among others), which have reported Upper Cretaceous ages. Based on this stratigraphic correlation we suggest that orogenesis may have begun in the Late Cretaceous with basin inversion, and then continued during the Cenozoic with basement thrusting.