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 and temporally associated with ore deposits and geothermal systems within the volcanic arc. Fault orientation and mechanical interaction among different fault systems influence the stress field in these arrangements, thus playing a first order control on the regional to local-scale fluid migration paths as documented by the spatial distribution of fault-vein arrays.Our selected case study is a Miocene-Pliocene hydrothermal system, that has a porphyry Type signal, that crops out in the precordillera of the Maule region along the Teno river Valley (ca. 35°S). Several regional to local faults were recognized in the field: (1) Two first-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) associated to 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 kinematic model of the structural development of the porphyry copper- system in the stepover between N-striking faults.The model yields the stress field within the stepover region and shows slip and potential opening distribution along the N-striking master faults under a regionally imposed stress field. We compare several scenarios based in the measure conditions and see how this affects the stress state. We see how σ_(1 )rotates counter-clockwise as it approaches to the main faults, and how the stresses evolves in the middle of the system where the main faults interact between each other, this, could lead to the generation of both NE- and NW-striking faults within the stepover area. Model results are consistent with the structural and kinematic data collected in the field attesting for enhanced permeability and fluid flow transport and arrest spatially associated with the stepover.