Mechanical interaction and localized fluid flow at a transtensional stepover as evidenced by a Miocene Andean porphyry copper system as a case study.

Margin-parallel and oblique fault systems in obliquely-convergent margins are associated with ore deposits and geothermal systems within the volcanic arc. Geomechanical interaction between faults perturbs the local stress field and exerts a first order control on hydrothermal fluid migration pathways as documented by the spatial distribution of fault-vein arrays, hydrothermal alteration, and magmatic intrusive bodies. We illustrate the case of a Miocene-Pliocene hydrothermal system with a porphyry-copper type signal (seemingly a barren system) in the precordillera of the Maule region of the Southern Andes along the Teno river Valley ( 35°S). Several faults are 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 between the first order faults, and (3) N40°-60°W striking subvertical, sinistral faults crossing the stepover zone. The regional and local geology is characterized by Eocene-Miocene volcano-sedimentary rocks associated with the andesitic Abanico Formation, intruded by dikes and Miocene granodioritic plutons. We develop a conceptual model to explain the structural development of the porphyry copper system in which the observed arrangement of second order faults is related to stress perturbations around the first order extensional stepover and implement a 2D Boundary Element Method (BEM) model to test the mechanical feasibility of variations of the conceptual model. The BEM model yields heterogeneous stress fields within the stepover region and slip and opening distributions along the N-striking master faults under a regionally imposed stress field. We compare several scenarios constrained by field observations. Our preferred model, in which deformation is driven by a regional N81°E σ₁ and suprahydrostatic fluid pressures in the stepover region, results in local clockwise rotation of σ₁ approaching the stepover region, matching the observed orientation and kinematics of secondary faults. Model results attest to enhanced permeability and fluid flow proximal to the stepover and the important role of inherited fault structures on crustal deformation in obliquely-convergent margins.