Dynamics and numerical modeling of deep-seated accretionary prisms in the Chilean Coastal Cordillera
Final Report Abstract
With the consistent application of a multidisciplinary process-oriented approach involving petrological, geochronological, structural geological and modelling techniques we were able to constrain the physical conditions of the development of an accretionary prism system at depths in comparison to deep-seated continental collision zones. The most conspicuous feature of the Chilean accretionary systems is a change in time from frontal to basal accretion mode through time involving the same accreted material. This can be detected over the entirely length of basement outcrop in the Chilean Coastal Cordillera from 29°S to 55°S even in regions that were strongly affected by postaccretional deformation. The phenomenon is also independent of the age of accretion. We detected a universal dehydration pattern in metapsammopelitic as well as metabasic rocks at low temperature responsible for this phenomenon implying that the subduction system cooled with time. Late subduction resulted in dehydration at higher temperature and pressure and hence deformation at greater depth and different mechanism than earlier subduction at warmer conditions. Numerical modelling shows that the formation of the accretion mode is independent of the type of convergent margin, i.e. extensional and compressional. We were able to reconstruct the conditions to form these two different states of plate coupling depending on convergence velocity and water propagation in the mantle wedge. Contrasting magma productivity occurs and the modelling results excellently match with examples from the Chilean convergent margin. U/Pb dating of detrital zircon from the Chilean accretionary prism that was derived from the entire South American continent and a combined study of its Hf isotopic signature revealed the specific history of crustal growth of the South American continent with three major periods of juvenile magma production at Late Archean, Paleoproterozoic (Transamazonian) and Late Mesoproterozoic (Grenvillian) times followed by periods of predominant crustal recycling. Whereas the deep-seated accretionary prisms in central Chile can be largely studied in its original state, strong deformational overprint is conspiceous north of about 34°S. Our structural studies of two model region enabled to distinguish syn- and postaccretional structures. This will be important for future studies in the more complicated basement areas in North Chile. Specific PT paths in frontal and basal accretion prisms as well as in a continental collision zone prior to the formation of the accretionary prism were derived and dated in selected model regions in southern and north-central Chile and W-Argentina respectively. These reflect the characteristic dynamic evolution of these three fundamental tectonic environments of convergent margins. At the Pacific margin of southern South America the collision with the microplate Chilenia occurred at 390 Ma and subsequently first subduction started at 340 Ma at Chilenia´s passive Pacific margin with formation of frontal accretionary systems. With cooling of the subduction system a change to basal accretion mode occurred at ca. 310 Ma. In South Chile a new cycle of accretion with formation of frontal accretionary prisms started in Jurassic times. During the project we could also make considerable methodological advances particularly with the PT-pseudosection technique that for the first time can successfully be applied particularly to very low and low grade conditions. Furthermore we were able to detect fundamental behaviour of white mica during age resetting of the Ar-Ar system particularly at low temperatures.
Publications
- (2007). Physical controls of magmatic productivity at Pacific-type convergent margins: Numerical modelling. Physics of Earth and Planet. Interior 163, 209-232
Gorczyk, W., Willner, A.P., Gerya, T.V., Connolly, J.A.D. & Jean-Pierre Burg, J.P.
- (2008). History of crustal growth and recycling at the Pacific convergent margin of South America at latitudes 29°–36° S revealed by a U–Pb and Lu–Hf isotope study of detrital zircon from late Palaeozoic accretionary systems. Chemical Geology 253, 114–129
Willner, A.P., Gerdes, A., Massonne, H.-J.
- (2008). Phase relations and dehydration behaviour of psammopelite and mid-ocean ridge basalt at very-low-grade to low-grade metamorphic conditions. Eur. J. Mineral. 20, 867-879
Massonne, H.-J. & Willner, A.P.
- (2009). Conditions and timing of pumpellyite-actinolite facies metamorphism in the Early Mesozoic frontal accretionary prism of the Madre de Dios Archipelago (50°20`S; S-Chile). J. Petrol. 50, 2127-2155
Willner, A.P., Sepúlveda, F.A., Hervé, F., Massonne, H.-J. & Sudo, M.
- (2009). Structural overprint of a late Palaeozoic accretionary system in north-central Chile (34°-35°S) during postaccretional deformation. Andean Geology 36, 17-36
Willner, A.P., Richter, P., Ring, U.
- (2011). Heterogeneous overprint of a late Palaeozoic fore-arc system in north-central Chile (32°–31°S) and radiometric dating by the Ar/Ar and fission track methods. Geol. Mag.
Willner, A.P., Massonne, H.-J., Ring, U., Sudo, M. & Thomson, S.
(See online at https://doi.org/10.1017/S0016756811000641) - (2011). Postaccretional transpressive shortening of a late Palaeozoic fore-arc system in north-central Chile (32°–31°S) Geol. Mag.
Ring, U., Willner, A.P. & Richter, P.P.
(See online at https://doi.org/10.1017/S0016756811000653) - (2011). The geodynamics of collision of a microplate ("Chilenia") in Devonian times deduced by the pressure-temperature-time evolution within part of a collisional belt (Guarguaráz Complex, W-Argentina). Contrib.Mineral.Petrol. 162, 303-327
Willner, A.P., Gerdes, A., Massonne, H.-J., Schmidt, A., Sudo, M., Thomson, S., Vujovich, G
