HR: 1340h
AN: V53A-0615    [Abstracts]
TI: Fault-Assisted Vertical Pluton Growth: Coastal Cordillera, North Chilean Andes
AU: * Grocott, J
EM: j.grocott@kingston.ac.uk
AF: School of Earth Sciences and Geography, Kingston University, Penrhyn Road, Kingston-upon-Thames, GU21 3PR United Kingdom
AU: Arevalo, C
EM: carevalo@sernageomin.cl
AF: Servicio Nacional de Geologia y Mineria, Avda. Santa Maria 0104, Santiago, 10465 Chile
AU: Welkner, D
EM: dwelkner@sernageomin.cl
AF: Servicio Nacional de Geologia y Mineria, Avda. Santa Maria 0104, Santiago, 10465 Chile
AU: Cruden, A
EM: cruden@utm.utoronto.ca
AF: Department of Geology, University of Toronto, 22 Russell St., Toronto, M5S 3B1 Canada
AB: Immense volumes of plutonic rocks exposed in magmatic arcs challenge our ability to understand fundamental interactions between deformation and magma emplacement at convergent margins. Although close temporal and spatial relationships between fault activity and emplacement of arc plutons have been inferred, the hypothesis that there is always a direct link between faults and plutons in magmatic arcs remains controversial. It is also remarkable that there is no consensus on how individual arc plutonic complexes were constructed. The current assumption is that large granitic plutons formed from large magma bodies but this is rejected here because it fails to account for the sheet-like form and composite nature of many arc plutons. We show that composite arc plutons in the Chilean Coastal Cordillera were constructed incrementally, unit-by-unit, and that dip-slip on reactivated, steeply-dipping faults was instrumental to this process. Extension to oblique-extension of the overriding plate at the Andean subduction boundary in Triassic to Palaeocene time was accommodated by displacement on margin-parallel fault systems that were initially extensional but were reactivated as strike-slip and later still, as contractional fault systems. As the retreating subduction boundary evolved, large volumes of mainly granitic magmas were emplaced into the upper plate. In the Vallenar district (29§S), elongate plutons with an asymmetrical, wedge-shaped cross section have one steeply-dipping side marked by synplutonic ductile fabrics that reworked a steeply-dipping fault. Partial-coupling across the fault during subsidence of the pluton floor caused a large-scale monocline to form in the host rocks, so that layering is characteristically deflected down towards the steep pluton margin. The vertical limb of these monoclines contains a high-temperature ductile shear zone with a down-dip stretching fabric and pluton-down shear sense. All of these features are expressions of fault-assisted vertical pluton growth. Symmetrical plutonic complexes, with a back-to-back wedge-shape in cross section, have a more cryptic relationship to margin-parallel faults. Fault-slip was still the important control on emplacement and equal amounts of floor subsidence in each fault sidewall led to a symmetrical pluton shape. Significantly, faults did not always propagate to the top of the complexes, and may remain unexposed, so that the link between pluton growth and faulting is then obscure in map view.
DE: 8035 Pluton emplacement
DE: 8100 TECTONOPHYSICS
DE: 8110 Continental tectonics--general (0905)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting