"Linked orogen-oblique fault zones in the central Argentine Andes : the basis of a new model for Andean orogenesis and metallogenesis"

Abstract

A major conjugate set of northwest and northeast oriented orogen-oblique fault/lineament zones has been identified in the Central Andes. These fault/lineament zones transect the orogen, extending for some 800km from the Chilean forearc to the Argentinian foreland. This study was initiated out of a desire to understand how these orogen-oblique fault/lineament zones had influenced the time-space development of the Andean orogen, and to quantify a spatial relationship that apparently exists between them, and the distribution of Tertiary mineralisation in the Andes.Study areas for detailed and reconnaissance mapping, 40K/40Ar and 40Ar/39Ar geochronology and apatite fission-track thermochronology, were selected along and about two major orogen-oblique fault zones in the Central Argentine Andes, the Tucuman, and Valle Fertil fault zones. Both fault zones meet in a confluence-zone of opposed-vergence reverse faults defining the present day western margin of the Sierras Pampeanas (contractional basement uplift), and the southern margin of the extensive Altiplano-Puna (high plain).Detailed structural analyses, including fault kinematic studies in local study areas, suggest that Tertiary strike-slip deformation on the Tucuman and Valle Fertil fault zones is transferred to dominantly reverse displacement approaching the apparent fault confluence-zone. These two faults are interpreted to link and interact at this point and the term linked fault complex is introduced to describe this region of fault interaction. Strain type varies longitudinally along the fault zones approaching the fault from oblate non-coaxial at some distance from the linkage zone, to oblate coaxial, through a plane strain null-point and to vertical prolate strains at the point of fault linkage and interaction. The linkage of these two faults defines a crustal-wedge that is converging westward with the main cordillera.Apatite fission-track thermochronology, structural studies and seismological evidence reveal that this crustal-wedge has been exhuming under east-west contractional conditions since the Upper Miocene-Pliocene.The Sierras Pampeanas crustal wedge has formed to the east of and following the development of a main cordillera and subsequent east-vergent foreland fold and thrust belt to its west. Contraction-driven exhumation of the Sierras Pampeanas is coeval with the initial waning and then total cessation of arc-magmatism in the cordillera to the west. Detailed mapping and reconnaissance work along the southern Altiplano- Puna to the north of the Tucuman Fault Zone has revealed a similar time-space relationship but with different relative and absolute timing.Observations from the southern Altiplano Puna and Sierras Pampeanas areas have led to the definition of three primary Andean tectonic time-space elements:1. Basement-involved contractional elements;2. thin-skinned contractional elements; and3. arc-magmatic elements.This study has revealed that these three primary tectonic elements are diachronous both across and along strike in the Central Andes.A new model for Andean orogenesis is developed that defines the process as a combination of these three elements. Major lateral thickening of the orogen develops as episodic increments of contraction driven basement-involved exhumation and uplift, occurring to the east of, and converging with an existing cordillera to the west. This east-west directed basement-involved contraction and uplift preferentially develops or reactivates an orthogonal network of orogen-oblique fault zones. Wrench and transpressional shear along the fault-zones transfers to reverse-faulting and the development of vertical prolate strains where they interact to form fault complexes.Thin-skinned eastward verging foreland fold and thrust belts define interim periods late-synchronous with and following major basement-involved episodes of lateral thickening in the orogen. Early-formed fold and thrust belts are eventually incorporated into the thickened orogen by the subsequent development and bulk westward translation of contraction-driven basement-involved uplifts to their east.Arc-magmatism in the central Andes develops along successive north-south oriented axes during periods alternating between episodes of lateral thickening and coeval with foreland fold and thrust belt development. The apparent eastward younging of successive arc-magmatic axes is interpreted to be a function of bulk westward translation and thrust overriding of the South American continent across the trench- axis.This study has demonstrated that there is a clear time-space relationship between the emplacement of intrusion-related mineralisation in the Andes and basement- involved lateral thickening episodes where crustal contraction overprints an immediately preceding arc-volcanic episode. Moreover, it is suggested that mineralisation and associated-intrusives may favour one of, bulk oblate or prolate strains and as such may be preferentially emplaced within an orogen-oblique fault- zone or within the macro-scale zone of influence of a fault complex.