Structural and numerical modeling of fluid flow and evolving stress fields at a transtensional stepover: A Miocene Andean porphyry copper system as a case study.

Abstract

Obliquely convergent subduction orogens show both margin-parallel and margin-oblique fault systems that are spatially andtemporally associated with ore deposits and geothermal systems within the volcanic arc. Fault orientation and mechanicalinteraction among different fault systems influence the stress field in these arrangements, thus playing a first-order control onthe regional to local-scale fluid migration pathways as documented by the spatial distribution of fault-vein arrays. Our selectedcase study is a Miocene-Pliocene hydrothermal system, that has a porphyry-copper type signal, that crops out in the precordilleraof the Maule region along the Teno river Valley (ca. 35°S). Several regional to local faults were recognized in the field: (1) Twofirst-order, N-striking subvertical dextral faults overlapping at a right stepover; (2) Second-order, N60°E-striking steeply-dipping,dextral-normal faults located at the stepover, and (3) N40°-60°W striking subvertical, sinistral faults crossing the stepover zone.The regional and local scale geology is characterized by volcano-sedimentary rocks (Upper Eocene- Lower Miocene) associatedwith the Abanico Formation, intruded by coeval dikes and Miocene granodioritic plutons (U-Pb zircon age of 18.2 ± 0.11 Ma).We implement a 2D Boundary Element Displacement Discontinuity Method (BEM) model to test the mechanical feasibility of apotential porphyry copper- system structural development, and its kinematic model consisting in two NS-striking faults developinga stepover. The model yields the stress field within the stepover region and shows slip and potential opening distribution along theN-striking master faults under a regionally imposed stress field. We compare several scenarios based on the measure conditionsand see how this affects the stress state. The model shows how rotates counter-clockwise as it approaches to the main faults,and how the stresses evolve in the overlapping zone where the main faults interact with each other. This, in turn, could lead tothe generation of both NE- and NW-striking faults within the stepover area. Model results are consistent with the structural andkinematic data collected in the field attesting for enhanced permeability and fluid flow transport and arrest spatially associatedwith the stepover. We thank the Project FONDECYT no. 1141139, and the Centro de Excelencia en Geotermia de los Andes (CEGA,FONDAP-CONICYT) for the economic contribution made.